Re-throw Fetch Errors to make them consistent

Different browsers format Fetch errors differently; give them some
sort of consistency by re-throwing something with a common string.

This will allows us to filter out fetch errors from sentry logging,
as well.

src/sidebar/services/api.js

@@ -136,6 +136,12 @@ function createAPICall(
           body: data ? JSON.stringify(stripInternalProperties(data)) : null,
           headers,
           method: descriptor.method,
+        }).catch(() => {
+          // Re-throw Fetch errors such that they all "look the same" (different
+          // browsers throw different Errors on Fetch failure). This allows
+          // Fetch failures to be either handled in particular ways higher up
+          // or for them to be ignored in error reporting (see `sentry` config).
+          throw new Error(`Fetch operation failed for URL '${url}'`);
         });
       })
       .then(response => {

src/sidebar/services/test/api-test.js

@@ -175,7 +175,8 @@ describe('sidebar.services.api', function () {
         // Network error
         status: null,
         body: 'Service unreachable.',
-        expectedMessage: 'Service unreachable.',
+        expectedMessage:
+          "Fetch operation failed for URL 'https://example.com/api/profile'",
       },
       {
         // Request failed with an error given in the JSON body