Fix input mismatch errors not reported the same way as Java in Python

[alimg]

I'm getting different error messages from Java and Python runtimes for the same grammar. Figured out a check from the Java version was missing in the Python one so here's it

[ericvergnaud]

Hi,
while you're at it, and since you have a grammar to reproduce the issue, could you add a test case in:
runtime-testsuite/test/org/antlr/v4/test/runtime/descriptors/ParserErrorsDescriptors.java
Eric

[alimg]

Hi Eric,
Added a test case which can reproduce the issue. Looks like we didn't notice before since the problems occurs when there are more terminal symbols defined.

[ericvergnaud]

Great!
Not sure what's wrong with the cpp tests, but clearly unrelated to this.
@parrt blessed

[parrt]

@alimg you need to sign contributors.txt

[alimg]

@parrt updated