Improve error message when no valid task is found

[fjtirado]

Currently for workflows that are not compliant with the spec, a long stack trace is printed as natural result of a failed parsing.
This long stack trace contains details of the failure, but they are expressed as java exception, which are sometimes difficulty to interpret.
It would be better (sacrificing some perfomance to increase usabillity) to validate the file againts the DSL schema before proceeding with the parsing. The validation will give the user, in one shot, all the format errors.