Clean up handling of files without a known encapsulation.

Fail with an "unsupported encapsulation" error for MTP2 and SSCOP,
rather than just returning "unknown encapsulation", and fail with that
if the encapsulation isn't filled in as well, although that might be a
deeper problem.

(Not that people should be handing text output files from K12 analyzers
anyway - they should hand us RF5 files and, if we can't handle their
file, file a bug and give us the file so we can further reverse-engineer
the format.)

Change-Id: I6bbd5f81787d69bd3b41eaedf2893d179f11ad6a
Reviewed-on: https://code.wireshark.org/review/5792
Reviewed-by: Guy Harris <guy@alum.mit.edu>