Need a clear error message when lemur finds no matches to the database

[ppreshant]

I am running tests with a tiny database and found that the error when lemur doesn't find any matches to the fastq file in the database could be clarified.

Relevant line lemur/671

I would add a if(n_reads == 0) ; error('none of the reads match the database') response here.

Currently this is the error I get

Command error:
  2025-08-26 04:32:08 PM INFO:  Starting run of minimap2 at 2025-08-26 16:32:08.851068
  2025-08-26 04:32:08 PM INFO:  Finished running minimap2 in 140.483 ms
  2025-08-26 04:32:09 PM INFO:  Finished loading taxonomy in 15.223 ms
  2025-08-26 04:32:09 PM INFO:  Finished initializing F in 0.139 ms
  2025-08-26 04:32:09 PM INFO:  Finished building alignment model in 0.065 ms
  2025-08-26 04:32:09 PM INFO:  Finished constructing P(r|s) in 18.008 ms
  Traceback (most recent call last):
    File "/home/pbk1/micromamba/other-envs/env-e0dcef690672920a7ce4d792d8222b07/bin/lemur", line 901, in <module>
      main()
    File "/home/pbk1/micromamba/other-envs/env-e0dcef690672920a7ce4d792d8222b07/bin/lemur", line 887, in main
      run.EM_complete()
    File "/home/pbk1/micromamba/other-envs/env-e0dcef690672920a7ce4d792d8222b07/bin/lemur", line 672, in EM_complete
      self.low_abundance_threshold = 1. / n_reads
  ZeroDivisionError: float division by zero