url: add descriptive error to UTS46 data load failure

Bug: 778929
Change-Id: I2bde041735eb48f69ead8230e785d37be2505cd8
Reviewed-on: https://chromium-review.googlesource.com/746527Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512941}

url/url_canon_icu.cc

@@ -101,7 +101,10 @@ struct UIDNAWrapper {
     // registrars, search engines) converge toward a consensus.
     value = uidna_openUTS46(UIDNA_CHECK_BIDI, &err);
     if (U_FAILURE(err)) {
-      CHECK(false) << "failed to open UTS46 data with error: " << err;
+      CHECK(false) << "failed to open UTS46 data with error: " << err
+                   << ". If you see this error message in a test environment "
+                   << "your test environment likely lacks the required data "
+                   << "tables for libicu. See https://crbug.com/778929.";
       value = NULL;
     }
   }