Simplify behavior of FieldResponse#getError to only return the first field error

[rstoyanchev]

While getErrors in FieldResponse simply returns all field errors, by contrast getError does a more exhaustive search that also includes global, request errors too, but only if the field is "failed" (i.e. has no value). So if the field has both a value and nested field errors, then getError would return null while getErrors would be non-empty.

We should align the two by making getError return the first field error. Separately, one still has the option to check if the field has a value or not, if the GraphQlResponse has global, request errors.

[rstoyanchev]

On second thought, the more logical thing is to deprecate getError() altogether.