Better message if judge answer file gets judge error

[Matistjati]

Improve error message from
WARNING judge answer file got JE [output validator validator (C++) crashed, status 134]
to
WARNING judge answer file got JE [output validator validator (C++) crashed, status 134] on testcase data/secret/group1/1.ans

Second is much more helpful. Code fragility:

• will throw an exception if there is no data in the path to the answer file. Should be impossible, as it won't be considered for being a testcase. Maybe symlinks can be weird? But iirc, we disallow symlinks going out of data.
• Might give weird results if you have a subfolder in the test data named "data". At that point, blame yourself

Also, why doesn't python have a rindex function??? This is more ugly than it should be..

[gkreitz]

Makes sense to indicate the test case here. Long-term, we should be a bit smarter about this particular warning (but that's for a later day - I jotted down thoughts in #344).

Ignoring the more long-term idea I jotted down there, this looks good, but just use the existing function in the f-string when printing instead of the computation you added to get a relative path. :)