Unexpected behaviour when bad input is provided to myst build CLI command

[fwkoch]

Description

Using version v1.1.55:

In the above screenshot I ran myst build --pdf index.pdf in a folder where index.pdf does not exist, nor is there an export defined that should create index.pdf. It printed a somewhat-unexpected error message "unrecognized extension" - I would expect it to either (1) interpret index.pdf as the input file and say "unrecognized file" or (2) interpret index.pdf as the output and be happy with .pdf (though (2) should be the case, output should only be under -o).

Further, after that error message, it still claims to perform index.pdf -> _build/exports/index.pdf which then actually does index.md -> _build/exports/index.pdf

Proposed solution

I think this is just an error in the logic of collecting exports provided by the CLI, rather than export frontmatter.