Improving read/write error messages and including default compiler

Error messages were little helpful, since they didn't include the problematic path: variables were added to the error messages to improve this.
Also, there should be no need for the developer to manually provide the compiler path if it's in the user path; thus, I've added "lessc" as a default value for the `compilerPath` attribute.
cleaning nodePath from the command if it's not given and printing the compiler return if there's an error in the proccess
tests the own output CSS file for write, and if it does not exist, create it
prints the lessc output on fail
concatenates 'write error' when lessc error code is 2 (it does not outputs anything, but it's a write error as of [lessc:230-231](https://github.com/cloudhead/less.js/blob/master/bin/lessc#L230)
Merge pull request #6 from igorsantos07/patch-1

Improving read/write error messages and including default compiler