Improve error messages of non-str displayname/avatar_url (#8705)

This PR fixes two things:

* Corrects the copy/paste error of telling the client their displayname is wrong when they are submitting an `avatar_url`.
* Returns a `M_INVALID_PARAM` instead of `M_UNKNOWN` for non-str type parameters.

Reported by @t3chguy.

changelog.d/8705.misc

@@ -0,0 +1 @@
+Improve the error returned when a non-string displayname or avatar_url is used when updating a user's profile.

synapse/handlers/profile.py

@@ -189,7 +189,9 @@ async def set_displayname(
                 )
 
         if not isinstance(new_displayname, str):
-            raise SynapseError(400, "Invalid displayname")
+            raise SynapseError(
+                400, "'displayname' must be a string", errcode=Codes.INVALID_PARAM
+            )
 
         if len(new_displayname) > MAX_DISPLAYNAME_LEN:
             raise SynapseError(
@@ -273,7 +275,9 @@ async def set_avatar_url(
                 )
 
         if not isinstance(new_avatar_url, str):
-            raise SynapseError(400, "Invalid displayname")
+            raise SynapseError(
+                400, "'avatar_url' must be a string", errcode=Codes.INVALID_PARAM
+            )
 
         if len(new_avatar_url) > MAX_AVATAR_URL_LEN:
             raise SynapseError(