Improve error message for version intersection conflicts

[chudnyi]

This PR enhances the error message when version intersection is not possible, making it easier to identify which packages are causing conflicts.

Before:

$ pkgx +nodejs.org@24 +openssl.org@3.6.0 openssl version
Error: no intersection possible

The error message was not informative, making it difficult to identify which packages were causing the conflict.

After:

$ pkgx +nodejs.org@24 +openssl.org@3.6.0 openssl version
Error: "no intersection possible for openssl.org: @3.6.0 and ^1.1"

The improved error message now includes:

• The package name causing the conflict
• The specific version constraints that couldn't be resolved

This allows users to much more quickly identify the root cause of the problem and fix it.

pkgx version: 2.7.0

[chudnyi]

Another example

Before

$ pkgx "+nodejs.org>=22.21.0" node -v
Error: invalid range description

After

$ pkgx "+nodejs.org>=22.21.0" node -v
Error: "invalid range description for nodejs.org: >=22.21 and *"

[jhheider]

this is a good fix, chudnyi. thank you!

[jhheider]

i moved the error cleanup a little higher up, so we don't have to push a &str through so many functions just to mutate an error message.

[coveralls]

coverage: 90.345% (-0.05%) from 90.397%
when pulling fc9d3da on chudnyi:feature/improve-intersection-error-message
into 6b45c9e on pkgxdev:main.