Wip/h3nk3 watch project files

The improved error message is only useful when there is a template that cannot be found (which happens everytime Activator is started with the Activator root project). Now it is cleared what the error means.

Apart from triggering a ``compile`` upon any source file changes this PR also has some minor refactoring improvements a la the boy scout rule.

The client side of this functionality, i.e. having a flag to toggle compilation on change or not, is __not__ part of this PR. Atm there will always be a recompilation on a file change. 

Further improvements is to check what type of source file it is and take action based on that, e.g. if it is a test file then ``test:compile`` should be invoked instead. As this logic will move into a sbt plugin there has been no such emphasis in this PR. 
no need for this to be a "var" right, you want to mutate the AtomicLong not make a new one