Catch last credential error from API

When we are unable to delete security key because it's the last
one for that user, API throws an error. Here we catch that error
and display useful message to the user.

Use security key instead of webauthn credential

in user facing message - for consistency and readability.

We use security key term in user facing stuff and webauthn
credential in the code.

app/main/views/user_profile.py

@@ -11,6 +11,7 @@ from flask import (
     url_for,
 )
 from flask_login import current_user
+from notifications_python_client.errors import HTTPError
 from notifications_utils.url_safe_token import check_token
 
 from app import user_api_client
@@ -283,8 +284,18 @@ def user_profile_manage_security_key(key_id):
 @main.route("/user-profile/security-keys/<uuid:key_id>/delete", methods=['POST'])
 @user_is_platform_admin
 def user_profile_delete_security_key(key_id):
-    user_api_client.delete_webauthn_credential_for_user(
-        user_id=current_user.id,
-        credential_id=key_id
-    )
+
+    try:
+        user_api_client.delete_webauthn_credential_for_user(
+            user_id=current_user.id,
+            credential_id=key_id
+        )
+    except HTTPError as e:
+        message = "Cannot delete last remaining webauthn credential for user"
+        if e.message == message:
+            flash("You cannot delete your last security key.")
+            return redirect(url_for('.user_profile_manage_security_key', key_id=key_id))
+        else:
+            raise e
+
     return redirect(url_for('.user_profile_security_keys'))