qa: remove error string checks and check w/ return value

qa: remove error string checks and check w/ return value

I ran into this failure once #54972 was merged. The test is validating
the error string returned due to the failed mount. There aren't any
return value checks - which is a _more_ important check. Generic error
string checks will fail once a (error) string is changed (typo, etc..).

Signed-off-by: Venky Shankar <vshankar@redhat.com>