Fix resolving relative paths with corral run

[mfelsche]

The logic to determine the absolute path to an executable was using FilePath.from(base, subdir) which errors if the resulting path is not below the base. This is the case if subdir is e.g. ../foo.

This fix stems from trying to execute:

$ corral run -- ../../ponyc/build/debug/ponyc
run: couldn't run program: ../../ponyc/build/debug/ponyc .

The error output is clearly not really informative.
With this PR the above command actually runs the existing ponyc.

Other changes in here all related to improving corral run:

• First try to resolve relative paths against the current working directory, then against $PATH. This enables running relative paths to parent folders (See the above command).
• Display different errors if no args were provided or the executable to run could not be found

[SeanTAllen]

So the path you gave was valid but it wasn't run? I'm confused by the explanation.

[mfelsche]

The path was valid and existing and runnable. The code to resolve relative paths had a flaw in that it used FilePath.from which errors when one tries to escape that path upwards using ... This is how FilePath ensures safety wrt to its capability. In this very case this safety net is actually hurting, so i switched to using Path.join instead.

[SeanTAllen]

Gotcha

[SeanTAllen]

Would you agree this is two changes? If yes, then please add one manually to change log and add two release notes entries. Then add a correct changelog label to this PR and rename the PR to match whatever the automatically added changelog entry should be.

We need a bot added changelog as without a changelog label, release notes are ignored.

[SeanTAllen]

@mfelsche this needs a changelog entry manually added for the Improved Error messages for corral run`` item. Can you do that then will be ready for merging. The changelog entry will be added for "Fix resolving relative paths with corral run" automatically