Better error messages

[roop]

Fixes #489.

All errors from accessing the server API (APIv3), or on parsing the data got from the server API (including JSON parsing errors), show the server URL.

@fkooman Some examples:

"/info" errors out:

"/connect" errors out:

Error parsing returned JSON:

Only the most used APIv2 stuff is updated -- the rest of APIv2's error messaging is left unimproved.

[ghost]

Awesome :heart: Thanks!