throw a better exception when environment variables are missing

Signed-off-by: Nico Schottelius <nico@bento.schottelius.org>

cdist/emulator.py

@@ -28,7 +28,14 @@ import sys
 import cdist
 from cdist import core
 
-class MissingEnvironmentVariable(cdist.Error):
+class MissingRequiredEnvironmentVariableError(cdist.Error):
+    def __init__(self, name):
+        self.name = name
+        self.message = "Emulator requires the environment variable %s to be setup" % self.name
+
+    def __str__(self):
+        return self.message
+
 
 class Emulator(object):
     def __init__(self, argv, stdin=sys.stdin.buffer, env=os.environ):
@@ -46,7 +53,8 @@ class Emulator(object):
             self.object_source  = self.env['__cdist_manifest']
             self.type_base_path = self.env['__cdist_type_base_path']
 
-        except KeyError:
+        except KeyError as e:
+            raise MissingRequiredEnvironmentVariableError(e.args[0])
 
         self.object_base_path = os.path.join(self.global_path, "object")