Add string err comparisons to backfill e2e

[akremsa]

What this PR does / Why we need it:
This PR improves tests output in case of a failure.

Before:

        	            	expected: *errors.errorString(&errors.errorString{s:"EOF"})
        	            	actual  : *status.Error(&status.Error{e:(*status.Status)(0xc000b08600)})

After:

        	Error:      	Not equal: 
        	            	expected: "EOF"
        	            	actual  : "qwe"