Decide expressiveness of UnicodeSet parsing errors

[skius]

How expressive should UnicodeSet parse errors be? Does it suffice if we show which character at which position was the issue, or should we give precise information also about what we expected? (i.e., "\xag<-- error: was parsing an \x-escape, expected precisely two hex-characters, got 'g'")

EDIT: Examples of current parse errors: https://github.com/unicode-org/icu4x/pull/3547/files#diff-1ab141f559ba2ebd644683b4cf5255a30d1e0a7f949b9cf950522f1a8b0cbcc5R1146