Enhance Fail parser with a custom error message

[byF]

It's useful to have a way how to specify a custom error message for the Fail parser; the current workaround is to throw an exception instead of using the Fail parser.

The implementation is backwards-compatible, the original Fail signature doesn't change and it works the same way as before.
The updated version assumes the user wants to have the access to the custom error message even if ctx.verboseFailures is not true.

Issue: closes #243

[byF]

Please let me know if you want me to change anything, @lihaoyi

[byF]

No interest?

[lihaoyi]

@byF sorry just got to this. Is there any reason you are passing in verboseFailure: Boolean = true as an argument? From the description, the only difference I would expect to see in the new overload would be the custom error message, and I'd expect it to use ctx.verboseFailure just like the original method does

[lefou]

Looks unrelated

[lefou]

Looks unrelated

[makkarpov]

Weirdly enough Fail(msg) version just ignores error message completely. And test is also written to catch default "fail" message, not custom one.