[core/errors] Add custom exceptions

Add custom exceptions and add error handling to the engine's module
loading logic. I.e. when a non-existent module is loaded, an exception
is thrown now.

see #23

bumblebee/error.py

@@ -0,0 +1,11 @@
+"""Collection of all exceptions raised by this tool"""
+
+class BaseError(Exception):
+    """Base class for all exceptions generated by this tool"""
+    pass
+
+class ModuleLoadError(BaseError):
+    """Raised whenever loading a module fails"""
+    pass
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4