Dont try to combine failed batches

[rjmackay]

This pull request makes the following changes:

• fix: Exclude failed/pending batches when combining batches …
  We were trying to include failed batches in the combine operation and so it failed when the filename didnt exist
• refactor: Mark batches as failed if any exception occurs during export

Test checklist:

• composer test
• I certify that I ran my checklist

Ping @ushahidi/platform

[coveralls]

Coverage decreased (-0.02%) to 20.513% when pulling 4812223 on hotfix/dont-try-to-combine-failed-batches into 2913f0c on master.