Generate better error messages for concat. with indefinite width.
Generate better error messages for concat. with indefinite width.

This patch generates a more user friendly error message for a
concatenation with an indefinite width operand.

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