Tweak in memory_error

Tweak in memory_error

This patch adds a local variable exception of type 'enum errors' and
pass it to throw_error, because 'err' is of type
'enum target_xfer_error', and we're abusing it to store an 'enum errors'.

gdb:

2014-02-07  Yao Qi  <yao@codesourcery.com>

	* corefile.c (memory_error): Get 'exception' from ERR and pass
	'exception' to throw_error.