Actually show the error message from an Error token

[JelteF]

Before the error output is like this:

(37, 13): unexpected token type in inline table: Error

With this change it's like this:

(37, 13): unexpected token type in inline table: keys cannot contain } character

[JelteF]

The travis failure seems to be caused by some network failure. I'm quite sure it should be fine when restarted.

[pelletier]

Good catch! Thank you for the patch!