Give a more useful error message if debugger is unable to attach to Dart app

[elliette]

Chrome currently prevents the debugger from attaching to "hosted apps", or any page with a "hosted app" in it (any app listed in chrome://apps).

Here are a couple Chrome bugs with details (linked to relevant comments):

• https://bugs.chromium.org/p/chromium/issues/detail?id=885025#c11
• https://bugs.chromium.org/p/chromium/issues/detail?id=1126825#c39

When this happens, the Dart Debug Extension shows the No Dart application found error, even though we have already detected that the app is a Dart app.

We should show a more relevant error, to help the user figure out what might be wrong.