`debug-files check` should search directory recursively

[szokeasaurusrex]

While investigating #2032, I discovered that when passed a path to a directory, debug-files upload recursively searches the directory for debug files. However, debug-files check treats the directory as if it were the debug file, and so the command fails with an "unsupported file" error, since a directory is clearly not a debug file that we would support.

Instead, we should recursively search the directory for debug files in the debug-files check command, to match the upload command's behavior. Ideally, we should reuse the recursive searching logic from the upload command.

Or, at a minimum, we should improve the error message when we check a directory. Something like the following could do:

Error: debug-files check can only check individual debug files, but the provided path leads to a directory. Please a pass path to an individual file, instead.