Error messages for invalid version = literal: ... in config

[rudyardrichter]

It's a very minor issue but an invalid version config field causes error messages that (imo) could be more specific/informative. If it would be welcome, I would happily add a PR to have an error suggest the correct format.

Missing variable name

As an example this configuration

version = literal: myproj/__init__.py

produces this error:

ValueError: not enough values to unpack (expected 3, got 2)

File available, but missing variable name

For example this configuration:

version = literal: myproj/__init__.py:

leads to this error:

scriv.exceptions.ScrivException: Couldn't find literal: 'literal: src/template_python/__init__.py:'

[nedbat]

Good idea!

Now (as of commit bf5d9cf) both of these errors will produce: ScrivException: Couldn't read 'version' configuration: Missing value name: 'literal: myproj/__init__.py'. I also added more detail to other kinds of failures reading literals.

[nedbat]

This is now released as part of scriv 1.2.1.