Improve shell errors handling (#121)

Improve shell errors reporting, so that vroom prints the actual shell errors.

vroom/shell.py

@@ -285,15 +285,16 @@ def __str__(self):
     return out.rstrip('\n')
 
 
-class FakeShellNotWorking(Exception):
+class FakeShellNotWorking(vroom.test.Failure):
   """Called when the fake shell is not working."""
 
   def __init__(self, errors):
     self.shell_errors = errors
     super(FakeShellNotWorking, self).__init__()
 
   def __str__(self):
-    return 'The fake shell is not working as anticipated.'
+    errors_description = '\n'.join(list(map(str, self.shell_errors)))
+    return 'The fake shell is not working as anticipated:\n' + errors_description
 
 
 class FakeShellFailure(vroom.test.Failure):