vscode extension: better status bar information

[Philipp15b]

Right now, when verification fails, the status bar shows "Verified". This is very confusing. Ideally, we'd distinguish between verification errors, type errors, syntax errors, timeouts, and successful verification.

In addition, the extension seems to show "Et tu Brute?" in the status bar if the verifier never started. This is the case if the server does not automatically start (which is unfortunately the default right now). If the server never started, we should print something like "Caesar not started".