Print info about request on error

This can help in diagnosing certain issues. For example, if I were
trying to use a "http+unix" URL but I don't have #299, then I'll get the
following:

    [marca@marca-mac2 httpie]$ http http+unix://%2Ftmp%2Fprofilesvc.sock/status/pid

    http: error: ConnectionError: ('Connection aborted.', gaierror(8, 'nodename nor servname provided, or not known'))
    while doing GET request to URL: http://http+unix//%2Ftmp%2Fprofilesvc.sock/status/pid

Having the URL in the error message is super useful here so that I know an
extra `http://` is getting prepended and it's not doing what I expected.

httpie/core.py

@@ -183,7 +183,13 @@ def main(args=sys.argv[1:], env=Environment()):
         #       Network errors vs. bugs, etc.
         if traceback:
             raise
-        error('%s: %s', type(e).__name__, str(e))
+        msg = str(e)
+        if hasattr(e, 'request'):
+            request = e.request
+            if hasattr(request, 'url'):
+                msg += ' while doing %s request to URL: %s' % (
+                    request.method, request.url)
+        error('%s: %s', type(e).__name__, msg)
         exit_status = ExitStatus.ERROR
 
     finally: