Improve the error message when the Windows server info are invalid

[codygarver]

At the moment, the error message is not very helpful "invalid argument" is very vague.

Bounty: https://www.bountysource.com/issues/7004041-improve-the-error-message-when-the-windows-server-info-are-invalid

[jeremypw]

There is a new(ish) connect server dialog now so it is uncertain whether this issue is satisfactorily addressed. Probably not, although there is no suggestion as to what suitable messages might be.

The new dialog might give two different messages, depending on the error made. Either "Connection refused" if the share or domain given do not exist or the user is not permitted to access them. Or "verify your details" if the user name, domain or password are missing - together with an icon in those entries needing attention.

In the former case, or when there is another serious error, the infobar just gives the error message returned by Gtk, which ought to be sufficient (but could be made more user friendly).

[jeremypw]

Will bear this in mind for the rewritten dialog in the Gtk4 version.