Improve exception when no credentials are found

[omus]

If you attempt to use aws_config() where there are no credentials setup you get a confusing error message:

ERROR: LoadError: LoadError: IOError(Base.IOError("connect: connection timed out (ETIMEDOUT)", -110) during request(http://169.254.169.254/latest/meta-data/iam/info))

Stacktrace:
 [1] wait_connected(::Sockets.TCPSocket) at /buildworker/worker/package_linux32/build/usr/share/julia/stdlib/v1.3/Sockets/src/Sockets.jl:520
 [2] connect at /buildworker/worker/package_linux32/build/usr/share/julia/stdlib/v1.3/Sockets/src/Sockets.jl:555 [inlined]
 [3] connect at /buildworker/worker/package_linux32/build/usr/share/julia/stdlib/v1.3/Sockets/src/Sockets.jl:541 [inlined]
 [4] #getconnection#14(::Bool, ::Int32, ::Base.Iterators.Pairs{Symbol,Any,Tuple{Symbol,Symbol,Symbol},NamedTuple{(:require_ssl_verification, :iofunction, :verbose),Tuple{Bool,Nothing,Int32}}}, ::typeof(HTTP.ConnectionPool.getconnection), ::Type{Sockets.TCPSocket}, ::SubString{String}, ::SubString{String}) at /home/omus/.julia/packages/HTTP/lZVI1/src/ConnectionPool.jl:568
 [5] #getconnection at ./none:0 [inlined]
 [6] #getconnection#11(::Int32, ::Int32, ::Int32, ::Int32, ::Bool, ::Base.Iterators.Pairs{Symbol,Union{Nothing, Int32},Tuple{Symbol,Symbol},NamedTuple{(:iofunction, :verbose),Tuple{Nothing,Int32}}}, ::typeof(HTTP.ConnectionPool.getconnection), ::Type{HTTP.ConnectionPool.Transaction{Sockets.TCPSocket}}, ::SubString{String}, ::SubString{String}) at /home/omus/.julia/packages/HTTP/lZVI1/src/ConnectionPool.jl:517
 [7] (::HTTP.ConnectionPool.var"#kw##getconnection")(::NamedTuple{(:reuse_limit, :iofunction, :verbose, :require_ssl_verification),Tuple{Int32,Nothing,Int32,Bool}}, ::typeof(HTTP.ConnectionPool.getconnection), ::Type{HTTP.ConnectionPool.Transaction{Sockets.TCPSocket}}, ::SubString{String}, ::SubString{String}) at ./none:0
 [8] #request#1(::Nothing, ::Type, ::Int32, ::Base.Iterators.Pairs{Symbol,Any,Tuple{Symbol,Symbol,Symbol},NamedTuple{(:iofunction, :verbose, :require_ssl_verification),Tuple{Nothing,Int32,Bool}}}, ::typeof(HTTP.request), ::Type{HTTP.ConnectionRequest.ConnectionPoolLayer{HTTP.StreamRequest.StreamLayer{Union{}}}}, ::HTTP.URIs.URI, ::HTTP.Messages.Request, ::Array{UInt8,1}) at /home/omus/.julia/packages/HTTP/lZVI1/src/ConnectionRequest.jl:41
 [9] (::HTTP.var"#kw##request")(::NamedTuple{(:iofunction, :verbose, :require_ssl_verification),Tuple{Nothing,Int32,Bool}}, ::typeof(HTTP.request), ::Type{HTTP.ConnectionRequest.ConnectionPoolLayer{HTTP.StreamRequest.StreamLayer{Union{}}}}, ::HTTP.URIs.URI, ::HTTP.Messages.Request, ::Array{UInt8,1}) at ./none:0
 [10] #request#1(::Base.Iterators.Pairs{Symbol,Any,Tuple{Symbol,Symbol,Symbol},NamedTuple{(:iofunction, :verbose, :require_ssl_verification),Tuple{Nothing,Int32,Bool}}}, ::typeof(HTTP.request), ::Type{HTTP.ExceptionRequest.ExceptionLayer{HTTP.ConnectionRequest.ConnectionPoolLayer{HTTP.StreamRequest.StreamLayer{Union{}}}}}, ::HTTP.URIs.URI, ::Vararg{Any,N} where N) at /home/omus/.julia/packages/HTTP/lZVI1/src/ExceptionRequest.jl:19
 [11] #request at ./none:0 [inlined]
 [12] #request#1(::VersionNumber, ::String, ::Nothing, ::Nothing, ::Base.Iterators.Pairs{Symbol,Integer,Tuple{Symbol,Symbol},NamedTuple{(:verbose, :require_ssl_verification),Tuple{Int32,Bool}}}, ::typeof(HTTP.request), ::Type{HTTP.MessageRequest.MessageLayer{HTTP.ExceptionRequest.ExceptionLayer{HTTP.ConnectionRequest.ConnectionPoolLayer{HTTP.StreamRequest.StreamLayer{Union{}}}}}}, ::String, ::HTTP.URIs.URI, ::Array{Pair{SubString{String},SubString{String}},1}, ::Array{UInt8,1}) at /home/omus/.julia/packages/HTTP/lZVI1/src/MessageRequest.jl:51
 [13] (::HTTP.var"#kw##request")(::NamedTuple{(:verbose, :require_ssl_verification),Tuple{Int32,Bool}}, ::typeof(HTTP.request), ::Type{HTTP.MessageRequest.MessageLayer{HTTP.ExceptionRequest.ExceptionLayer{HTTP.ConnectionRequest.ConnectionPoolLayer{HTTP.StreamRequest.StreamLayer{Union{}}}}}}, ::String, ::HTTP.URIs.URI, ::Array{Pair{SubString{String},SubString{String}},1}, ::Array{UInt8,1}) at ./none:0
 [14] http_request(::Dict{Symbol,Any}) at /home/omus/.julia/packages/AWSCore/nRYAp/src/http.jl:42
 [15] http_get(::String) at /home/omus/.julia/packages/AWSCore/nRYAp/src/http.jl:80
 [16] macro expansion at /home/omus/.julia/packages/Mocking/hFQQ4/src/mock.jl:29 [inlined]
 [17] _ec2_metadata(::String) at /home/omus/.julia/packages/AWSCore/nRYAp/src/AWSCredentials.jl:263
 [18] ec2_instance_credentials() at /home/omus/.julia/packages/AWSCore/nRYAp/src/AWSCredentials.jl:282
 [19] #AWSCredentials#3(::Nothing, ::Type{AWSCore.AWSCredentials}) at /home/omus/.julia/packages/AWSCore/nRYAp/src/AWSCredentials.jl:146
 [20] Type at ./none:0 [inlined]
 [21] aws_config() at /home/omus/.julia/packages/AWSCore/nRYAp/src/AWSCore.jl:115
...

This seems not very user friendly.