This adds a new invalid instruction that is used for encoding
implicit throws that are emitted by the compiler. This makes it
possible to distinguish such runtime errors from user-provided,
explicit throws.
Unimplemented features moved to their own exception
InternalCompilerError is an exception that really should be reserved for
actual internal errors of the compiler. Unimplemented features can now
use either solUnimplemented( ) or, if it should be conditional, then
solUnimplementedAssert( ).
* Revert some unimplemented exceptions, add handlers
The jsonCompiler and CommandLineInterface needed handlers for the
new UnimplementedFeatureException, and some cases I had moved on to
the new exception were better treated as real internal compiler
errors.
* Standardize on "Unimplemented feature" message
