improve grpc-fallback browser-test

[leahecole]

Separating out from #1769 - the it 'should handle an error' test in browser-test/test/test.grpc-fallback.ts does not properly assert on the error message, and doing so is going to require some gymnastics to adjust setMockFallbackResponse. @sofisl and I experimented a bit today and we can note that

• When we tried constructing an EchoClient and calling the echo method like the "makes a request" test, we do not properly get an error
• When reverting the test to its previous use of sinon, we see a SymbolIterator error
• When we try to assert on the error message right now, we get a "Fail to fetch" error from gaxios - it seems that the mock fallback response function doesn't properly pass the adapter all the way down to gaxios when it's being called from the stub, rather than from the method itself