Improve code generation user-facing error on non-existent source file

[fleimgruber]

When referencing a non-existent source file ("non-existing.xsd"):

xsdata generate non-existing.xsd --package some.models -ss single-package

only a CodeGenerationError is shown:

========= xsdata v24.2.1 / Python 3.10.8 / Platform win32 =========

Traceback (most recent call last):
...
xsdata.exceptions.CodeGenerationError: Nothing to generate.

when instead a FileNotFoundError would be more informative for the user.

[fleimgruber]

@tefra If you think this is a good change then I will create a PR for it.

[tefra]

The transformer is very lenient and doesn't fail if an included schema can't be retrieved, but yes I agree that behavior shouldn't be extended to the main resource file

[fleimgruber]

@tefra could you give me a quick heads up please on where this is best tested? I looked at the testing and exceptions seem to be captured and printed to the user? Assuming that is true, I went to test for process output (i.e. not testing with assertRaises) of runner invocation results and could not find the expected traceback containing xsdata.exceptions.CodeGenerationError as in OP, but only the line ========= xsdata v24.2.1 / Python 3.10.8 / Platform win32 =========. Please see also fleimgruber@41ce482.

[tefra]

A proper warning will now be generated

❯ xsdata generate i/dont/exists.wsdl
========= xsdata v24.2.1 / Python 3.11.4+ / Platform linux =========

Warnings: 1
warning: Resource not found file:///home/chris/projects/xsdata/i/dont/exists.wsdl
~/projects/xsdata nicer-cli-errors *3 ❯