Unimplemented features moved to their own exception (#1361)

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

solc/CommandLineInterface.cpp

@@ -604,6 +604,12 @@ bool CommandLineInterface::processInput()
 			 << boost::diagnostic_information(_exception);
 		return false;
 	}
+	catch (UnimplementedFeatureError const& _exception)
+	{
+		cerr << "Unimplemented feature:" << endl
+			 << boost::diagnostic_information(_exception);
+		return false;
+	}
 	catch (Error const& _error)
 	{
 		if (_error.type() == Error::Type::DocstringParsingError)