Expose reqwest's inner error in the fetch function (#424)

* Expose reqwest's inner error in the `fetch` function

We were converting the `reqwest::Error` type directly to `span::Error`
and the rendering chain stopped at that. This ignored the
inner (source) error which contained the most useful information to
figure out what went wrong.

This gets us from:

    Error: error: failed to fetch: error: error decoding response body

to:

    Error: error: failed to fetch: error: error decoding response body: missing field `discord` at line 16049 column 1

* Generate a spanned::Error directly from the inner JSON error

The code is shorter and less hacky. The end-user message looks much
better too.

* Use `to_string()` instead of `format!`

* Move the question mark out of `Ok(json_response?)`

Author: tomassedovic

crates/rust-project-goals/src/team.rs

@@ -122,6 +122,14 @@ where
     // dropped.
     in_thread(|| {
         let url = format!("{}/{}", v1::BASE_URL, path);
-        Ok(reqwest::blocking::get(&url)?.json()?)
+        let json_response = reqwest::blocking::get(&url)?.json().map_err(|e| {
+            use std::error::Error;
+
+            e.source()
+                .map(|json_error| spanned::Error::str(json_error.to_string()))
+                .unwrap_or(e.into())
+        })?;
+
+        Ok(json_response)
     })
 }