interpreter: Allow traps to include custom error strings (#1236)
This means we can give more precise/useful errors for runtime failures.
Change interp::Result from an enum to struct so it can hold the
result enum plus an optional detailed error message.
Add TRAP_MSG and TRAP_IF_MSG macros that work just like TRAP and
TRAP_IF but contain a format string and printf-like arguments which
are formatted to produce the error message.
