Make it more clear when the browser page crashes

[natebosch]

Currently if a web test causes the browser page to crash (like a memory leak causing an OOM error) then we'll report the test as "did not complete", but it's not clear why.

If there were no tests that passed before the crash it also erroneously prints "No tests were found".

00:14 +0: compiling test/some_test.dart                                                    
Info: Compiling with sound null safety
Compiled 10,728,431 characters Dart to 1,210,643 characters JavaScript in 13.0 seconds using
 317.645 MB of memory
Dart file /tmp/dart_test_MAXCHX/runInBrowser.dart compiled to JavaScript: /tmp/dart_test_ETR
MRK/test_GNQJGL/some_test.dart.browser_test.dart.js

00:35 +0: some test - did not complete [E]                                                 
00:35 +0: Some tests failed.                                                               
No tests were found.

If we can detect this happening, perhaps by seeing that the channel closes before expected, then we should give an indication of the situation in the failure message.