unhandled rejection if .bidsignore is a directory

[rwblair]

trafficstars

In 1.7.3 and 1.8.1 validator will fail with EISDIR error trying to read .bidsignore as if it were a file. We should fail better here.

Found an old install of 1.3.4 on the HPC and interestingly it did not fail in this case.