Fix: Unwrap nested errors in error handler for Crystal::Error

[straight-shoota]

This addresses number 2. of #12874 to make sure information about the original error is not being hidden.

$ echo 'require "bar"' > foo.cr
$ echo '{% `filedoesnotexist` %}' > bar.cr
$ crystal build foo.cr
Error: while requiring "./bar"
$ bin\crystal build foo.cr
Error: while requiring "./bar"
Error: Error executing process: 'filedoesnotexist': The system cannot find the file specified.

This is a very simple implementation and probably not the best finished solution that we could find. But it does the job and it enables a basic requirement. Without this change, unexpected errors (such as the File::NotFoundError when a system command fails on Windows, #12874) are basically impossible to debug.

[sdogruyol]

Thank you @straight-shoota 🙏