Improve error messages in case of invalid file paths or URIs

This closes #170
Make sure TaskManager does not accept task deployments when shutting down
[FLINK-1198] Broadcast variables are shared between tasks in the same TaskManager
[FLINK-1198] Make Broadcast variables shared per taskmanager, rather than a shared singleton per JVM
             (supports multiple TaskManagers in the same JVM)
[FLINK-1198] Broadcast variables can share initialization via broadcast variable initializers