Improving error feedback

[ccbrown]

Creating a single issue for the various issues where feedback can be improved:

• pkg-config is missing
• git is missing
• git can't establish the authenticity of the host
• git is denied permission to the remote
• unused or misplaced parameters

[vmrob]

• [ ] Warn when pkgconfig files reference directories outside of build_directory
• [ ] When Needy changes from repository to directory, it doesn't delete the previous source folder leading to os exceptions during the build.

[vmrob]

git is missing

Should be covered by #112