Fixed Error Handling for Async Fetcher Calls

* `GTMSessionFetcher`’s async `beginFetch()` method returns either data or error, but not both (unlike the completion handler-variant)
* Without the data, `processedResponseData(from:error:)` has less details and can’t categorize the error correctly
* However, the same details can be retrieved from the error’s user info

FirebaseFunctions/Sources/Functions.swift

@@ -536,7 +536,7 @@ enum FunctionsConstants {
       if error.domain == kGTMSessionFetcherStatusDomain {
         localError = FunctionsError(
           httpStatusCode: error.code,
-          body: data,
+          body: data ?? error.userInfo["data"] as? Data,
           serializer: serializer
         )
       } else if error.domain == NSURLErrorDomain, error.code == NSURLErrorTimedOut {