Do not extend backoff reporting useless errors (ampproject#6565)

These include already reported errors (which somehow got thrown again),
cancelled loading messages, and now a large majority of loading errors.

If we had a whole bunch of loading errors, and then a real honest to god
bug, we'd almost never receive the bug because the "reported" loading
errors would delay sending the message for so long.

src/error.js

@@ -51,10 +51,10 @@ self.AMPErrors = accumulatedErrorMessages;
  * @param {function()} work the function to execute after backoff
  * @return {number} the setTimeout id
  */
-let globalExponentialBackoff = function(work) {
- // Set globalExponentialBackoff as the lazy-created function. JS Vooodoooo.
- globalExponentialBackoff = exponentialBackoff(1.5);
- return globalExponentialBackoff(work);
+let reportingBackoff = function(work) {
+ // Set reportingBackoff as the lazy-created function. JS Vooodoooo.
+ reportingBackoff = exponentialBackoff(1.5);
+ return reportingBackoff(work);
 };
 
 /**
@@ -187,11 +187,11 @@ function reportErrorToServer(message, filename, line, col, error) {
  }
  const url = getErrorReportUrl(message, filename, line, col, error,
  hasNonAmpJs);
- globalExponentialBackoff(() => {
- if (url) {
+ if (url) {
+ reportingBackoff(() => {
  new Image().src = url;
- }
- });
+ });
+ }
 }
 
 /**