runalyzer-gather: check if file exists before creating tables

[matthiasdiener]

Otherwise, a strange error message is shown:

before:

$ runalyzer-gather out.sqlite mpi*sqlite
Scanning...          [########################################] ETA ?
Traceback (most recent call last):
  File "/Users/mdiener/Work/emirge/miniforge3/envs/sqlite/bin/runalyzer-gather", line 57, in <module>
    main()
  File "/Users/mdiener/Work/emirge/miniforge3/envs/sqlite/bin/runalyzer-gather", line 53, in main
    gather_multi_file(outfile, infiles, fmap, qmap, fg, features, dbname_to_run_id)
  File "/Users/mdiener/Work/emirge/logpyle/logpyle/runalyzer_gather.py", line 246, in gather_multi_file
    db_conn.execute("create table runs ({})".format(",".join(run_columns)))
sqlite3.OperationalError: table runs already exists

after:

$ runalyzer-gather ../logpyle/out.sqlite mpi*sqlite
Scanning...          [########################################] ETA ?
Error: output file '../logpyle/out.sqlite' already exists, exiting.