Throw a more descriptive error if the metadata can't be found

If we can't read the DDC metadata file we get an odd state error:

```
Thread 0 main threadStateError: Bad state: No element
at List.last(growable_array.dart:255)
at AppInspector._initialize(inspector.dart:90)
at (async)
at AppInspector.initialize(inspector.dart:155)
at (async)
at ChromeProxyService.createIsolate(chrome_proxy_service.dart:180)
at (async)
```

This is because we get an empty list of libraries. We should implement better logging here and throw a better more descriptive state error.

Start at AppInspector._initialize in inspector.dart, where the stack trace shows List.last failing because the libraries list is empty. Trace the DDC metadata read path and improve the logging and state error so the missing metadata condition is described clearly.

Written by the indexing model from the issue text.