Refactor API error handling

Use a declarative style for mapping errors. Also add a new one for an
"invalid request" message.

wideq/core.py

@@ -85,6 +85,13 @@ class TokenError(APIError):
         pass
 
 
+class InvalidRequestError(APIError):
+    """The server rejected a request as invalid."""
+
+    def __init__(self):
+        pass
+
+
 class MonitorError(APIError):
     """Monitoring a device failed, possibly because the monitoring
     session failed and needs to be restarted.
@@ -95,6 +102,13 @@ class MonitorError(APIError):
         self.code = code
 
 
+API_ERRORS = {
+    "0102": NotLoggedInError,
+    "0106": NotConnectedError,
+    9000: InvalidRequestError,  # Surprisingly, an integer (not a string).
+}
+
+
 def lgedm_post(url, data=None, access_token=None, session_id=None):
     """Make an HTTP request in the format used by the API servers.
 
@@ -123,13 +137,12 @@ def lgedm_post(url, data=None, access_token=None, session_id=None):
     # Check for API errors.
     if 'returnCd' in out:
         code = out['returnCd']
-        if code != '0000':
-            message = out['returnMsg']
-            if code == "0102":
-                raise NotLoggedInError()
-            elif code == "0106":
-                raise NotConnectedError()
+        if code == '0000':
+            pass
+        elif code in API_ERRORS:
+            raise API_ERRORS[code]()
         else:
+            message = out['returnMsg']
             raise APIError(code, message)
 
     return out