avro

AVRO-3999 - Avoid warnings in Perl test suite (#2953)

* Add error message when schema do not match in Perl

When the schema did not match in a call to Avro::BinaryDecoder::decode,
a Avro::Schema::Error::Mismatch error was thrown without a body. This
was generating a warning when trying to stringify an undefined value,
and resulted in the empty string being used as the error message, which
was not veryb informative.

This change adds a message which should solve both issues.

* Do not exit sub via next in Perl tests

This silences a loud warning in xt/schema.t