Distinguish between Composer downloaded or error

In _checkComposerPhar(), make error message distinguish between error executing composer.phar and if it exists at all.

On my host, phar was not added to suhosin whitelist, so composer.phar wouldn't run. cakephp-composer only said that Composer wasn't installed, but the error was actually that composer failed to run.
Merge pull request #9 from eithe/master

Distinguish between Composer downloaded or error