Fix AttributeError on config changes on Ubuntu.

Fixes #390.
Fix popping of a dead question in prompter.

This happens when a question for a file path for an already cancelled download
gets popped.

Fixes #415.
Add a warning for manual edits to the config.

Closes #373.
Move debug log to bottom in crash dialog.

Closes #365.
Disable report button in crash dialog on :report.
Make it possible to enter a keymode twice.

If we don't allow this, we can get stuck e.g. when doing this:

- Press m to get a quickmark prompt.
- Click a javascript prompt button.
- Exit the javascript prompt.
- We have the quickmark prompt open but we're in normal mode.
Remove unnecessary if-branch in fuzzy_url.

The first branch already checks for `os.path.exists(path)`, so it doesn't make
sense for the second one to check that again (ANDed with some other condition).
Handle an invalid cwd properly.

Fixes #370.
Fix name collision.
Log full exception on restart errors.
Handle :restart correctly with Python eggs.

Fixes #323.