Alchemist should fail clearly when no incarnation is available

[paagamelo]

Actual behaviour

At present, if Alchemist is launched and no incarnation is available (e.g. due to the fact that no incarnation module has been imported) the error message produced will look like this:

ERROR it.unibo.alchemist.Alchemist - No valid launchers.
ERROR it.unibo.alchemist.Alchemist - HeadlessSimulationLauncher: Alchemist headless runner requires a simulation file

Which does not mention the fact that no incarnation is available. If we try to run a simulation, e.g. using the protelis incarnation, we get something like this:

Exception in thread "main" java.lang.IllegalStateException: protelis is not a valid incarnation. Supported incarnations are: []

Followed by a stack trace.

Desired behaviour

When loading the main class, the classpath should be scanned looking for incarnations. If no incarnation is found:

• a warning should be produced if the user requested to print a help message
• a clear error, without stack trace and explaining how to solve the issue, should be produced in case the user tried to run Alchemist