Make backend argument type errors more specific

amanuensis/errors.py

@@ -8,10 +8,10 @@ class AmanuensisError(Exception):
 
 
 class ArgumentError(AmanuensisError):
-    """An internal call was made with invalid arguments"""
+    """An internal call was made with invalid arguments."""
 
 
-class BackendArgumentTypeError(AmanuensisError):
+class BackendArgumentTypeError(ArgumentError):
     """
     A call to a backend function was made with a value of an invalid type for the parameter.
     Specify the invalid parameter and value as a kwarg.