fix: make sure the engine executable exists

[leofracca]

Exit right away if the engine isn’t found.
This makes the error message clearer by saying the executable doesn’t exist, rather than showing the previous generic one: "Fatal; <engine_name> engine startup failure: Couldn't start engine process."

[Disservin]

This has multiple flaws
a) the filesystem usage is currently not guarded
b) this assume cmd is an exact path which
might not be the case when used together with dir
c) we had this in the past but removed it because we allowed engines to be started from PATH which means filesystem checks will fail unless the path is expanded which I don't want to do

[leofracca]

I think all three issues you mentioned can be addressed:
a) guard the std::filesystem logic
b) check if dir is defined, and concatenate with cmd before checking (maybe with an helper function and call it at the end of parseEngine and parseEach or in sanitize as it was done before #869?)
c) only run the check if we have an absolute path or dir is set

Would this be okay?

[Disservin]

yeah so for
a) you'd have to use NO_STD_FILESYSTEM, which currently also used in other places
b) that logic should go in sanitize
c) we can check if it is an absolute path, im not sure how you do the check when dir is set since you might have to check the path..

[leofracca]

Pushed a new commit with the following fixes:
a) guards for filesystem
b) added dir handling and move the logic to sanitize
c) check if using absolute path