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

lang/perl/lib/Avro/BinaryDecoder.pm

@@ -67,7 +67,7 @@ sub decode {
     my $type = Avro::Schema->match(
         writer => $writer_schema,
         reader => $reader_schema,
-    ) or throw Avro::Schema::Error::Mismatch;
+    ) or throw Avro::Schema::Error::Mismatch 'schema do not match';
 
     my $meth = "decode_$type";
     return $class->$meth($writer_schema, $reader_schema, $reader);

lang/perl/xt/schema.t

@@ -23,7 +23,7 @@ use File::Find;
 use_ok 'Avro::Schema';
 
 sub parse {
-    next unless /\.avsc$/;
+    return unless /\.avsc$/;
     open(my $fh, '<', $_);
     local $/ = undef;
     my $schema = <$fh>;