hawc2 io now prints better error messages

wetb/hawc2/Hawc2io.py

@@ -340,6 +340,7 @@ class ReadHawc2(object):
     ############################################################################
     # get items
     def __getitem__(self, key):
+        orig_key = key
         old_key = []
         while key in self.alias.keys() and not key in old_key:
             old_key.append(key)
@@ -348,6 +349,8 @@ class ReadHawc2(object):
             raise Exception('Circular alias')
         if isinstance(key,str) and key.isdigit():
             key=int(key)
+        if not isinstance(key,int):
+            raise KeyError("Cannot resolve a signal with the key '"+str(orig_key)+"'")
         return self.get_signal(key)