Improve error messaging

[KCMertens]

There are some places where errors (from blacklab, or from general web traffic/reachability issues) are not reported very nicely.
Go over this once more and improve the situation. There should be a clear error message to the user, with some extra logging in the console indicated what went wrong. (or perhaps when debug mode is enabled, a stack trace).

• When BlackLab response doesn't parse into json or xml (such as an error page from an intermediate proxy) it's printed verbatum.
• Display stack traces when BlackLab returns one #459
• Unhelpful message "Server returned at: CORPUSNAME/hits" #420