Add specific error message that creating Junctions requires absolute path

[SteveL-MSFT]

PR Summary

Creating a Junction requires an absolute path but currently provides a confusing error message. Fix is to check if the target path is absolute and return a specific error.

PR Context

Fix #18251

PR Checklist

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[pull-request-quantifier-deprecated]

This PR has 12 quantified lines of changes. In general, a change size of upto 200 lines is ideal for the best PR experience!

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Quantification details

Label      : Extra Small
Size       : +12 -0
Percentile : 4.8%

Total files changed: 3

Change summary by file extension:
.cs : +3 -0
.resx : +3 -0
.ps1 : +6 -0

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This pull request has been automatically marked as Review Needed because it has been there has not been any activity for 7 days.
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🎉v7.4.0-preview.4 has been released which incorporates this pull request.:tada:

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