Update the error message for invalid variables

[Juanadelacuesta]

Description

Currently the message returned when a storing a variable fails is:

Error saving nomad/jobs/my-job
Invalid name. Name must contain only alphanumeric or "-", "_", "~", or "/" characters, and be fewer than 128 characters in length.

Update it to be more inclusive, so it expresses that not only the name can be the error.

Fixes: #26234

Testing & Reproduction steps

Links

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• Changelog Entry If this PR changes user-facing behavior, please generate and add a
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• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
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  and job configuration, please update the Nomad website documentation to reflect this. Refer to
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  change requires notes within the upgrade guide.

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[pkazmierczak]

LGTM!

[Juanadelacuesta]

@aklkv Can you give me a hand over here?

[tgross]

I've rebased this to pick up test fixes.