Leftwm-theme should throw an error if the prebuilt themes aren't available for copying

[mautamu]

leftwm-theme/src/operations/new.rs

Line 72 in 22b537b

"0" => copy_files("/usr/share/leftwm/themes/basic_lemonbar/", &dir),

introduces an error where someone can create a new theme as a fork of a basic template but not see an error.

[mautamu]

Repro:

• install leftwm and leftwm-theme
• remove (or don't install) the prebuilt themes to /usr/share/leftwm/themes/
• create a theme patterned after one of the basics, e.g. polybar
• see bug

US: As a user, I want to see only the theme templates which are available to be cloned so that I can build my own themes quickly and easily.

Acceptance criteria:

• Only themes which are installed to /usr/share/leftwm/themes/ are provided as options.
• Attempting to select other themes is not possible and/or throws a clear error.