Make IDNA creation code CHECK on nullptr

Instead of set the pointer to nullptr during error,
CHECK the error w/ u_errorName for better diagnostic.

Bug: 950322
Change-Id: I9b0c0ba0d26a375a35a8e9c5df3431398c193e42
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1623465Reviewed-by: Jungshik Shin <jshin@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665698}

components/url_formatter/url_formatter.cc

@@ -344,8 +344,12 @@ struct UIDNAWrapper {
     // TODO(jungshik): Change options as different parties (browsers,
     // registrars, search engines) converge toward a consensus.
     value = uidna_openUTS46(UIDNA_CHECK_BIDI, &err);
-    if (U_FAILURE(err))
-      value = nullptr;
+    CHECK(U_SUCCESS(err)) << "failed to open UTS46 data with error: "
+                          << u_errorName(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.";
   }
 
   UIDNA* value;