Use sentinel errors

[ahmedalhulaibi]

The errors return should not have newline characters and should be defined as sentinel errors (either as constants or interface) so one can assert against them.

As an example of somewhere where this is being done:

flatfile/ffptag.go

Line 64 in ee3c85f

return errors.Errorf("flatfile.parseFlatfileTag: Invalid formatting of named option '%v'\nNamed options should be in the form option=value\nValid options:%v", options, validOptions)

---

Tests should also assert the specific error returned.

Example of a test not checking the error message returned at all:

flatfile/ffptag_test.go

Lines 60 to 70 in ee3c85f

	func TestFfpTagParseMissingParamErr_parseFfpTag(t *testing.T) {
	testVal := `flatfile:""`
	err := parseFlatfileTag(testVal, &flatfileTag{})
	if err == nil {
	t.Error("parseFfpTag should return missing parameter error")
	}
	t.Log(testVal)
	t.Log(err)
	}