Bug 1618011 part 9. Make error reporting for unions nicer. r=peterv

Instead of something like
"ByteStringSequenceSequenceOrByteStringByteStringRecord" we should have
"(sequence<sequence<ByteString>> or record<ByteString, Bytestring>)" in error
messages.

Please take a careful look at the resulting error messages (see attachment
in the bug) to see whether this makes sense.

Differential Revision: https://phabricator.services.mozilla.com/D64890