test(bigtable): Return error in correct format from test proxy

[bhshkh]

Before fix:

=== RUN   TestReadModifyWriteRow_NoRetry_TransientError
    test_workflow.go:133: The RPC to test proxy encountered error: rpc error: code = Unavailable desc = ReadModifyWriteRow failed
    readmodifywriterow_test.go:278: 
        	Error Trace:	cloud-bigtable-clients-test/tests/readmodifywriterow_test.go:278
        	Error:      	Should NOT be empty, but was <nil>
        	Test:       	TestReadModifyWriteRow_NoRetry_TransientError
    readmodifywriterow_test.go:279: 
        	Error Trace:	cloud-bigtable-clients-test/tests/readmodifywriterow_test.go:279
        	Error:      	Not equal: 
        	            	expected: 14
        	            	actual  : 0
        	Test:       	TestReadModifyWriteRow_NoRetry_TransientError
--- FAIL: TestReadModifyWriteRow_NoRetry_TransientError (0.01s)
FAIL
exit status 1
FAIL	github.com/googleapis/cloud-bigtable-clients-test/tests	0.345s

After fix i.e. returning error in status field of result instead of separate variable:

=== RUN   TestReadModifyWriteRow_NoRetry_TransientError
--- PASS: TestReadModifyWriteRow_NoRetry_TransientError (0.01s)
PASS
ok  	github.com/googleapis/cloud-bigtable-clients-test/tests	1.260s