Add message for unsupported release of part/bit select.
Add message for unsupported release of part/bit select.

This patch adds a real error message instead of an assert when
someone attempts to release a part or bit select. I plan to work
on the real functionality soon.

themperek/iverilog,CastMi/iverilog,CastMi/iverilog,themperek/iverilog,CastMi/iverilog,themperek/iverilog