[RISCV] Improve validation of opcode for .insn.

The lower 2 bits of the opcode must be 0x3. If the lower 2 bits are
0-2, it's a compressed instruction.

Merge 3 slightly different error messages into 1 to reduce code. The
messages differed slightly depending on whether we parsed a string
or an expression. The message gets a little more generic, but is no
more generic than what binutils prints.