3.0 - Project Task - Fixes and windows compatibility

Sorry for suppressing errors, I know that's not very nice, but apart from testing against the paths defined for the `open_basedir` option that seems to be the only way to avoid being flooded with errors on systems that restrict PHP filesystem acceess.
Let me quote my commit comment and explain that

> In case a path in the `PATH` environment variable contains a directory
named `composer`, it might be use as the alleged composer "executable"
as `file_exists` returns `true` for directories.

Say you have the paths `/some/path` and `/some/path/composer` (in which the actual composer executable may live) in your `PATH` environment variable. In that case `file_exists` would return `true` when checking `/some/path`, as it will test for `/some/path/composer` on it, which exists, but is a directory, thus executing the composer command will fail.