Check output filename extensions before writing

[cjordan]

I did something a little silly, but it exposed a poor error message:

# 1090008640.ms
birli *gpubox* -m *.metafits -u 1090008640.ms
# prints out
thread 'main' panicked at 'couldn't initialise uvfits writer: IO(Os { code: 21, kind: IsADirectory, message: "Is a directory" })', src/main.rs:1036:14

(I'm using the v0.5.0 branch and it failed on the line containing .expect("couldn't initialise uvfits writer"))

This error happens because the "file" already exists, and Rust notes that this is a directory when trying to initialise the output as a file. It would be good to have existing files checked and handled in this case, but also user input filenames checked; .ms files write as MS, .uvfits write as uvfits, etc.

[d3v-null]

I agree that this would be handy, and we could probably copy the code from Hyperdrive that does this.

At least the error message is a bit nicer since Marlu 0.6.0 MWATelescope/Marlu@01d2393 , this has been merged in for release in v0.7.0

cannot create directory, path={path} already exists and is not a directory