Fix error message for missing <inputdir>/<genome>.md5 file

[peterjc]

Fix error message for missing <inputdir>/<genome>.md5 file.

In this case triggered by running pyani createdb and then pyani anim without indexing the FASTA files.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality not to work as expected)
• This change requires a documentation update
• This is a documentation update

Action Checklist

• Work on a single issue/concept (if there are multiple separate issues to address, please use a separate pull request for each)
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• Work on this branch
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  • new code is commented, especially in hard-to-understand areas
  • corresponding changes to documentation have been made
  • tests for the change have been added that demonstrate the fix or feature works
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