guile disassembly hardcode TARGET_XFER_E_IO

guile disassembly hardcode TARGET_XFER_E_IO

Instead of adding a cast at the memory_error call, as needed for C++,
and have the reader understand the indirection, make it simple and
hardcode the generic memory error at the memory_error call site.

gdb/ChangeLog:
2015-11-17  Pedro Alves  <palves@redhat.com>

	* guile/scm-disasm.c (gdbscm_disasm_read_memory): Return -1 on
	error instead of TARGET_XFER_E_IO.
	(gdbscm_disasm_memory_error): Always pass TARGET_XFER_E_IO to
	memory_error.