Make errors about email password resets much clearer

changelog.d/5616.misc

@@ -0,0 +1 @@
+Make runtime errors surrounding password reset emails much clearer.

synapse/config/emailconfig.py

@@ -112,13 +112,17 @@ def read_config(self, config, **kwargs):
             missing = []
             for k in required:
                 if k not in email_config:
-                    missing.append(k)
+                    missing.append("email." + k)
+
+            if config.get("public_baseurl") is None:
+                missing.append("public_base_url")
 
             if len(missing) > 0:
                 raise RuntimeError(
-                    "email.password_reset_behaviour is set to 'local' "
-                    "but required keys are missing: %s"
-                    % (", ".join(["email." + k for k in missing]),)
+                    "Password resets emails are configured to be sent from "
+                    "this homeserver due to a partial 'email' block. "
+                    "However, the following required keys are missing: %s"
+                    % (", ".join(missing),)
                 )
 
             # Templates for password reset emails
@@ -156,13 +160,6 @@ def read_config(self, config, **kwargs):
                 filepath, "email.password_reset_template_success_html"
             )
 
-            if config.get("public_baseurl") is None:
-                raise RuntimeError(
-                    "email.password_reset_behaviour is set to 'local' but no "
-                    "public_baseurl is set. This is necessary to generate password "
-                    "reset links"
-                )
-
         if self.email_enable_notifs:
             required = [
                 "smtp_host",