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
