Make it clearer when stack installs to a spot that's not on the PATH or shadowed on the PATH

[mgsloan]

We already have a warning about this, but it gets lost in the mix:

Installing library in
/tmp/stack-upgrade17612/stack/.stack-work/install/x86_64-linux/lts-3.7/7.10.2/lib/x86_64-linux-ghc-7.10.2/stack-0.1.7.0-1qCQzKMpF5B2dxtC0o3K2h
Installing executable(s) in
/tmp/stack-upgrade17612/stack/.stack-work/install/x86_64-linux/lts-3.7/7.10.2/bin
Registering stack-0.1.7.0...
Completed all 2 actions.
Installation path /home/mgsloan/.local/bin not found in PATH environment variable
Copying from /tmp/stack-upgrade17612/stack/.stack-work/install/x86_64-linux/lts-3.7/7.10.2/bin/stack to /home/mgsloan/.local/bin/stack

Copied executables to /home/mgsloan/.local/bin:
- stack

There's a Installation path /home/mgsloan/.local/bin not found in PATH environment variable hiding in there. I'd rather see:

Completed all 2 actions.
Copying from /tmp/stack-upgrade17612/stack/.stack-work/install/x86_64-linux/lts-3.7/7.10.2/bin/stack to /home/mgsloan/.local/bin/stack

Copied executables to /home/mgsloan/.local/bin:
- stack

WARNING: Installation path /home/mgsloan/.local/bin not found in PATH environment variable.

There's also the case where the installation path is on the PATH, but some earlier dir in PATH also contains an executable with this name (shadowing). In this case, I think it makes sense to output something like WARNING: The "stack" executable found on the PATH environment variable is /usr/bin/stack, and not the version that was just installed. This means that "stack" calls on the command line will not use this version.