read errors need highlighting too

[dyoo]

WeScheme currently isn't doing a great job at highlighting read-based errors. The following erroneous code:

 (define hex-digits 
  (vector #\0 #\1 #\2 #\3
          #\4 #\5 #\6 #\7
          #\8 #\9 #\a #\b
          #\c #\d #e #\f))

gives the slightly unhelpful error message

<definitions>:98:18: read: no digits

with no highlighting. But we know exactly where the error is showing up at least, though we don't necessarily know how long.

[dyoo]

(The program error is the #e thing. But WeScheme should make that easier to see.)

[dyoo]

It's not being multi-color highlighted, but the "stack trace" does highlight it.