Better error message for unknown start of token

The span was fixed at some point to point to the correct character, but
the error message is still bad. Update it to emit the actual character
in question (potentially escaped).

Fixes #3747.
auto merge of #8550 : kballard/rust/token-start-err-msg, r=catamorphism

The span was fixed at some point to point to the correct character, but
the error message is still bad. Update it to emit the actual character
in question (potentially escaped).

Fixes #3747.