[jvm] CI error message should be more helpful

[nedtwigg]

When run in an IDE, the snapshot usually shows up as a diff. But on CI, it just says

org.opentest4j.AssertionFailedError: Snapshot mismatch

It would be way better to at least mark the first difference, something like:

Snapshot mismatch starting at Line 10:
- this was the old line 10
+ this is the new line 10

Need to figure out the other cases too (pure addition, pure removal, etc).

[nedtwigg]

• Implemented in Better error messages #477