Return error when last config file is non existent or has some error

[Gautham-coder]

Proposed changes

Added check if the last config file was read successfully, if not return error.

This PR addresses Issue: zowe/zss#716

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

PR Checklist

Please delete options that are not relevant.

• If the changes in this PR are meant for the next release / mainline, this PR targets the "staging" branch.
• My code follows the style guidelines of this project (see: Contributing guideline)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes
• video or image is included if visual changes are made
• Relevant update to CHANGELOG.md
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works, or describe a test method below

Testing

$ ./configmgr -s /some/valid/schema -p "FILE(/existing/file):FILE(/nonexistant/file)" env /output/file.env

Expected output:
ZWEL0318E - failed to read '/nonexistant/file.yaml' - EDC5129I No such file or directory.
ZWEL0319E - Failed to load configuration, element may be bad, or less likely a bad merge.

Unit Test:
Tested with configmgr //zowe.jfrog.io/zowe/libs-snapshot-local/org/zowe/configmgr/2.17.0-YAML-NON-EXISTENT/configmgr-2.17.0-2024062654.pax"

Working as expected.

[JoeNemo]

Approved.