Fix invalid variable names in error messages

[ptitzler]

Replace incorrect variable names in two error messages related to object storage authentication processing. AWS_IAM_ROLES_FOR_SERVICE_ACCOUNTS is an internal Elyra identifier, whereas AWS_WEB_IDENTITY_TOKEN_FILE is the variable name that should have been reported as problematic.

What changes were proposed in this pull request?

See above.

How was this pull request tested?

• No functional change.

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[elyra-bot]

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[kevin-bates]

LGTM