Fix MDSAL nonexistent namespace error being too restrictive

All that is needed is that the response contains a
<rpc-error> tag and that it mentions the bogus namespace.
Insisting on having also "does-not-exist" or other texts in
the error message is superfluous and easily leads to
spurious failures.

Change-Id: I202f4bf2a81573106763136a0d9bf4cc1bb5d8a2
Signed-off-by: Jozef Behran <jbehran@cisco.com>