EnemyManager: improve error message
EnemyGroupManager: fix parseEnemyFilesAndStore missing the first enemy

This makes sure to always assign a type to the 'new_enemy', which then
makes sure to have a correct '_categories' class member.
FileParser: document the behavior of new_section
EnemyGroups: allow exact amount as well as [min, max] on maps
Fix breaking out of loop when spawning enemy groups
Maprenderer: Fix wrong condition on inner loop
Merge pull request #797 from stefanbeller/fixenemy_groups

Fixenemy groups missing the type of the first enemy