add fallback when sequences cannot be downloaded

[rvhonorato]

Sometimes it is not possible to access the efetch endpoint for unknown reasons, this would raise an error and not output would be produced. In this PR I added a fallback that if the efetch endpoint raises an exception, the sequence identifiers are written to a file and a warning message is displayed with further instructions on how to proceed:

 [2023-03-15 12:03:06,867 117 WARNING] Could not fetch the fasta sequences, dumping the sequence IDs instead.
 [2023-03-15 12:03:06,867 120 WARNING] This is probably due to the NCBI server being inaccessible. Please try again later or manually download the sequences from NCBI
 [2023-03-15 12:03:06,867 123 WARNING] Please upload GH43_1_15032023.txt to `https://www.ncbi.nlm.nih.gov/sites/batchentrez` to download the sequences