Add compilation error when default enum value does not exist

[slemesle]

When Selma generates the custom enum mapping, it does not checks for validity of the default value if defined.
So we get a compilation warning on the generated class.

We should add a compiler check for that to report a comprehensive error.

[slemesle]

Selma will now raise a compilation error like that:

Invalid default value for @EnumMapper(from=fr.xebia.extras.selma.beans.EnumA.class, to=fr.xebia.extras.selma.beans.EnumB.class, default="c") fr.xebia.extras.selma.beans.EnumB.c does not exist