Installing shell utility silently fails

I've done a little debugging, and the exception handler in `[MPHomebrewSubprocessController runWithCompletionHandler:]` catches a `NSInvalidArgumentException` with `launch path not accessible` reason, even though Homebrew is installed on my machine at the default `/usr/local/bin/brew`.

Start by locating MPHomebrewSubprocessController runWithCompletionHandler: and trace how it launches /usr/local/bin/brew. Reproduce the launch path not accessible exception and inspect the exception handler's behavior. Done means the shell utility installation no longer silently fails and the relevant failure is surfaced or handled correctly.

Written by the indexing model from the issue text.