Handle memory write errors on gdb.base/break-always.exp

Handle memory write errors on gdb.base/break-always.exp

This is another case of the testcase not handling memory write errors that
happen on some targets (QEMU) when GDB attempts to modify an address that
should contain a breakpoint, for example.

The following patch handles this and prevents spurious failures from
happening. It also adds a foreach loop to avoid duplication of code
and hardcoded patterns.

gdb/testsuite/ChangeLog:

2015-04-29  Luis Machado  <lgustavo@codesourcery.com>

	* gdb.base/break-always.exp: Abort testing if writing to memory
	causes an error.