Switch compiler to _lliby_signal_error

This makes runtime casts to subtypes and record classes throw an
exception instead of triggering a fatal error. The most delicate part of
this is unwinding any partially consumed allocations. At the same time
actually pass through an irritant in GenCastCellToSubtype to ease
debugging.
Print exception strings unquoted

Just print the string's raw UTF-8 data instead of printing the message
cell as a datum. This prevents it from being quoted and escaped which is
a bit easier to read.