internal/driver: return an error on failed source fetching

[odeke-em]

This change inverts checks to ensure that we return a wrapped fs.ErrNotExist error if we completely fail to find a source fetcher, instead of panicking.

Fixes #863

[odeke-em]

Kind /cc @aalexand @ghemawat

[aalexand]

I sent #864 earlier today.

[odeke-em]

Oh cool, thanks @aalexand! Could we perhaps copy over and adapt the isolated test that I've crafted to prevent regressions?

func TestFetchDoesNotPanicOnMissingSource(t *testing.T) {
	_, _, err := fetch("non-existent-source", 10*time.Millisecond, 10*time.Millisecond, nil, http.DefaultTransport)
	if err == nil {
		t.Fatal("expected a non-nil error")
	}
	if g, w := err, fs.ErrNotExist; !errors.Is(g, w) {
		t.Fatalf("mismatched type: got %T, want %T", g, w)
	}
}

[aalexand]

Oh cool, thanks @aalexand! Could we perhaps copy over and adapt the isolated test that I've crafted to prevent regressions?

func TestFetchDoesNotPanicOnMissingSource(t *testing.T) {
	_, _, err := fetch("non-existent-source", 10*time.Millisecond, 10*time.Millisecond, nil, http.DefaultTransport)
	if err == nil {
		t.Fatal("expected a non-nil error")
	}
	if g, w := err, fs.ErrNotExist; !errors.Is(g, w) {
		t.Fatalf("mismatched type: got %T, want %T", g, w)
	}
}

My PR added a test already.

[odeke-em]

Alright, thanks!