Provide descriptive error when Simple Replay is ran without audit log being available.

[pvbouwel]

Issue #, if available: Related to https://github.com/awslabs/amazon-redshift-utils/issues/546 (no exact match but other code paths seem to have been taken care of already.

Description of changes: When no audit log is available an IndexError is thrown which is not clear for all users. This minor change will explain in a critical message that logs are not available yet and should be enabled and awaited.

Note that this code path is only hit if there is not a single useractivitylog file in the audit prefix. If there is a useractivitylog file then simple replay will correctly identify if there are no records for the provided time range.

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