More precise errors for bad unicode characters

[c42f]

Ported from equivalent errors in the reference parser

Part of #270

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (main@9a796bf). Click here to learn what that means. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #286   +/-   ##
=======================================
  Coverage        ?   96.73%           
=======================================
  Files           ?       14           
  Lines           ?     4038           
  Branches        ?        0           
=======================================
  Hits            ?     3906           
  Misses          ?      132           
  Partials        ?        0           

Impacted Files	Coverage Δ
src/kinds.jl	80.59% <ø> (ø)
src/parse_stream.jl	96.40% <100.00%> (ø)
src/tokenize.jl	98.88% <100.00%> (ø)