[flang][NFC] Change how error symbols are recorded

When an error is associated with a symbol, it was marked with a flag
from Symbol::Flag. The problem with that is that you need a mutable
symbol to do that. Instead, store the set of error symbols in the
SemanticsContext. This allows for some const_casts to be eliminated.

Also, improve the internal error that occurs if SetError is called
but no fatal error has been reported.

Differential Revision: https://reviews.llvm.org/D86740