♻️ Clean up InvalidPluginConfiguration interface

Added explicit message parameter that can be accessed, instead of
having to rely on exc.args[0] which *could* be empty. Now you must
instantiate this exception with a message argument.

The message is still forwarded to the parent exception in case other
usages still rely on exc.args[0], but this will be refactored later.

src/openforms/contrib/brk/service.py

@@ -1,4 +1,4 @@
-from openforms.forms.models.form import Form
+from openforms.forms.models import Form
 from openforms.plugins.exceptions import InvalidPluginConfiguration
 
 from .checks import BRKValidatorCheck
@@ -10,7 +10,7 @@ def check_brk_config_for_addressNL() -> str:
     if any(form.has_component("addressNL") for form in live_forms):
         try:
             BRKValidatorCheck.check_config()
-        except InvalidPluginConfiguration as e:
-            return e.args[0]
+        except InvalidPluginConfiguration as exc:
+            return exc.message
 
     return ""

src/openforms/contrib/kadaster/service.py

@@ -1,5 +1,5 @@
 from openforms.contrib.kadaster.config_check import BAGCheck
-from openforms.forms.models.form import Form
+from openforms.forms.models import Form
 from openforms.plugins.exceptions import InvalidPluginConfiguration
 
 
@@ -20,7 +20,7 @@ def check_bag_config_for_address_fields() -> str:
 
     try:
         BAGCheck.check_config()
-    except InvalidPluginConfiguration as e:
-        return e.args[0]
+    except InvalidPluginConfiguration as exc:
+        return exc.message
 
     return ""

src/openforms/plugins/exceptions.py

@@ -1,5 +1,7 @@
 class InvalidPluginConfiguration(Exception):
-    pass
+    def __init__(self, message: str, *args, **kwargs):
+        self.message = message
+        super().__init__(message, *args, **kwargs)
 
 
 class PluginNotEnabled(Exception):