Improve error when guest binary not found

[ludfjig]

When the given GuestBinary is not found, the error now includes the actual path that wasn't found.
Also:

• Removes a call to try_exists, which is unnecessary since canonicalize will already fail if the file doesn't exit.
• Removes an unnecessary recreation of matched pattern for GuestBinary::Buffer.
• Converts PathBuf to String in a more ideomatic way and removes potential panic if PathBuf cannot be converted to String (i.e. not utf8 encoded)