Exception should be thrown when an environment does not exist

[Meijuh]

The propel:build-sql task and other tasks have the default environment set to dev. When such an environment does not exists the task will simply output [phingcall] Error importing platform/Platform.php. Which is very obscure. I think the plugin should check if an environment given exists (including the default dev environment). If it does not exists something has to written to the std err and exit > 0.

[willdurand]

ping @rozwell