Exit with > 0 status code if pip fails to install due to existing build dir

[benbeanfield]

When pip fails to install a package due to the existence of a pre-existing build directory, it should exit with a status code that is greater than 0, in order to indicate that the command was unsuccessful.

$ pip install -U foobar
Downloading/unpacking foobar from https://pypi.example.com/foobar/foobar-0.1.1.tar.gz

pip can't proceed with requirement 'foobar from https://pypi.example.com/foobar/foobar-0.1.1.tar.gz' due to a pre-existing build directory.
 location: /opt/example/foo/venvs/foo/build/foobar
This is likely due to a previous installation that failed.
pip is being responsible and not assuming it can delete this.
Please delete it and try again.

[qwcode]

this was fixed in v1.5 (PR #1162)