display exceptions in threadpool, and look for 'mpich', not 'mpich2'

[rustandruin]

Two changes:

1. The mpich2 plugin searches for 'mpich2' in the output of 'update-alternatives --list mpirun' to find the location of the mpich2 path, but in Ubuntu 15.04, the command name is mpirun.mpich to run mpich2, so the search fails and causes an error. I changed the search for the more general 'mpich'

2. To facilitate debugging of plugins, prints the exceptions that arise in running a threadpool before raising a threadpool exception (the latter is not verbose enough in its output). I find this useful for seeing exactly what the commandline was in a node.ssh.execute command that failed during the run of a plugin.