Raise ExMake.ScriptError if an executable cannot be found.
Sanitize the ExMake.Logger interface.

Functions prefixed with `log_` are now exported. These respect the
`--loud` setting instead of always printing. The non-prefixed variants
are not exported and are primarily meant for internal use.
Remove some now-unnecessary checks from the Erlang/Elixir libraries.
Add an ExMake.Logger.log_result/1 function that outputs cyan text.
Print result messages when locating executables.