[fml][embedder] Improve thread-check logging (flutter#47020)

Developers working their app's runner code or in the native parts of
plugins are often required to call methods from the platform thread, or
face a crash.
Makes a minor improvement to fml::ThreadChecker's
IsCreationThreadCurrent log message.
This patch originally proposed to emit a much more specific error
message when calling methods that must be called on the platform thread
(typically originating in calls from plugins), however, given that we
don't ship debug engines as part of the SDK, this is really only going
to be useful to engine developpers, or people brave enough to run with
their own local engine build, in any case. 

Issue: flutter/flutter#135345

No changed tests since no functional change, just an error message
cleanup.

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Objective-C, Java style guides].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [ ] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
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fml/memory/thread_checker.h

@@ -75,9 +75,10 @@ class ThreadChecker final {
       if (0 == pthread_getname_np(current_thread, actual_thread,
                                   buffer_length) &&
           0 == pthread_getname_np(self_, expected_thread, buffer_length)) {
-        FML_DLOG(ERROR) << "IsCreationThreadCurrent expected thread: '"
-                        << expected_thread << "' actual thread:'"
-                        << actual_thread << "'";
+        FML_DLOG(ERROR) << "Object referenced on a thread other than the one "
+                           "on which it was created. Expected thread: '"
+                        << expected_thread << "'. Actual thread: '"
+                        << actual_thread << "'.";
       }
     }
 #endif  // __APPLE__