fix(models): better error messages than the built in Django ones for a few of the fields

Fixes #848
Fixes #849

Author: helgi

rootfs/api/models/domain.py

@@ -7,7 +7,12 @@
 class Domain(AuditedModel):
     owner = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.PROTECT)
     app = models.ForeignKey('App', on_delete=models.CASCADE)
-    domain = models.TextField(blank=False, null=False, unique=True)
+    domain = models.TextField(
+        blank=False, null=False, unique=True,
+        error_messages={
+            'unique': 'Domain is already in use by another application'
+        }
+    )
     certificate = models.ForeignKey(
         'Certificate',
         on_delete=models.SET_NULL,

rootfs/api/models/key.py

@@ -21,7 +21,12 @@ class Key(UuidAuditedModel):
 
     owner = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
     id = models.CharField(max_length=128)
-    public = models.TextField(unique=True, validators=[validate_base64])
+    public = models.TextField(
+        unique=True, validators=[validate_base64],
+        error_messages={
+            'unique': 'Public Key is already in use'
+        }
+    )
     fingerprint = models.CharField(max_length=128, editable=False)
 
     class Meta: