z80/disassembler: call memory_error_func when appropriate

If a call to the read_memory_func fails then we should call the
memory_error_func to notify the user of the disassembler of the
address that was a problem.

Without this GDB will report all memory errors as being at address
0x0.

opcodes/ChangeLog:

	* z80-dis.c (fetch_data): Call memory_error_func if the
	read_memory_func call fails.