Warn on 'cargo bin' or 'cargo install' with custom prefix

[Wilfred]

As discussed in #64, the following does not work:

$ evm config ~/evm
$ evm install emacs-24.5-bin
$ evm bin emacs-24.5-bin
/home/wilfred/evm/emacs-24.5-bin/bin/emacs
$ /home/wilfred/evm/emacs-24.5-bin/bin/emacs -Q
Warning: arch-dependent data dir `/usr/local/evm/emacs-24.5-bin/libexec/emacs/24.5/x86_64-unknown-linux-gnu/': No such file or directory
Warning: arch-independent data dir `/usr/local/evm/emacs-24.5-bin/share/emacs/24.5/etc/': No such file or directory
Warning: Lisp directory `/usr/local/evm/emacs-24.5-bin/share/emacs/24.5/lisp': No such file or directory
Error: charsets directory not found:
/usr/local/evm/emacs-24.5-bin/share/emacs/24.5/etc/charsets
Emacs will not function correctly without the character map files.
Please check your installation!

I think this should be a warning or even an error. Could evm install refuse to install -bin versions if a custom prefix has been set?