Allow error to be silenced through django config setting. (#33)

* Report error via return value.

* Add note to README.md.

README.md

@@ -139,6 +139,14 @@ class MobileSerializer(Serializer):
 2.  the argument `secret_key` of field
 3.  request.context["recaptcha_secret_key"]
 
+### Silence the check error
+
+If you need to disable the error, you can do so using the django settings.
+
+```python
+SILENCED_SYSTEM_CHECKS = ['drf_recaptcha.checks.recaptcha_system_check']
+```
+
 ## reCAPTCHA v3
 
 Validation is passed if the score value returned by Google is greater than or equal to required score.

drf_recaptcha/checks.py

@@ -1,5 +1,5 @@
 from django.conf import settings
-from django.core.checks import Tags, Warning, register
+from django.core.checks import Tags, Error, Warning, register
 from django.core.exceptions import ImproperlyConfigured
 
 from drf_recaptcha.constants import TEST_V2_SECRET_KEY
@@ -15,9 +15,11 @@ def recaptcha_system_check(app_configs, **kwargs):
 
     secret_key = getattr(settings, "DRF_RECAPTCHA_SECRET_KEY", None)
     if not secret_key:
-        raise ImproperlyConfigured("settings.DRF_RECAPTCHA_SECRET_KEY must be set.")
+        errors.append(
+            Error("settings.DRF_RECAPTCHA_SECRET_KEY must be set."),
+        )
 
-    if secret_key == TEST_V2_SECRET_KEY:
+    elif secret_key == TEST_V2_SECRET_KEY:
         errors.append(
             Warning(
                 "Google test key for reCAPTCHA v2 is used now.\n"