[Subcommands] Ensure an adjacent binary exists before using it

[doinkythederp]

This pull request updates the handling of subcommands in the Swift Driver to ensure correct behavior when the subcommand executable is not adjacent to swift-driver itself. Now, if the adjacent binary doesn't exist, the driver will fall back to a $PATH-based search rather than attempting to run the missing executable (this seems to be the intended behavior based on the comment above this code).

I also expanded the error message for the failure case here.

This change is important because allows users to use subcommands that didn't come bundled with Swift.

Fixes #1932

[artemcm]

@swift-ci test

[doinkythederp]

The error message for Windows seems to be about Predicate's string formatting, but I didn't change code related to that. Is there anything I need to do here to resolve the CI error? This is my first time contributing to Swift, so I don't really know the procedure.