Improve error message when module does not exist

[terminalmage]

This also simplifies the if/elif/else logic when the module does not exist.
The else would never be reached anyway since we are doing an
if X is not None followed by an if X is None.

This gives different and meaninful errors when a non-existant module is used,
as well as when a nonexistant function in an existing module is used.

[root@cent6 ~]# salt cent6 salt_util.sync_modules
cent6:
    Module 'salt_util' is not available.
[root@cent6 ~]# salt cent6 saltutil.sync_modulessss
cent6:
    'saltutil.sync_modulessss' is not available.

Fixes #22958

[twangboy]

@msteed Looks like @terminalmage took care of this.