Module doesn't handle deleted credentials

[ghost]

Bug description
If you verify credentials once, save them to the database, and delete them in Smaily dashboard the credentials will still seem valid in admin view. The user will not be notified that they are using deleted credentials, even when refreshing the page. Since apiCall returns nothing if it encounters an error, some functions which send data (customer sync/abandoned cart) may throw errors if request data is not validated. Abandoned cart drop-down list will not be populated with autoresponders.

Steps for reproducing
Steps to reproduce the behavior:

1. Install module
2. Go to Smaily dashboard and generate a fresh pair of credentials.
3. Go to module settings.
4. Validate credentials and save settings.
5. Delete the credentials in Smaily dashboard.
6. Refresh module settings page/Save settings again

Expected behavior
User is notified that saved credentials are invalid and module will not work

Actual behavior
No visual notifications that credentials are invalid. Running customer sync and/or abandoned cart may throw an error.

Versions
1.5.6.4 - Smaily for OpenCart 1.2.0
2.2.0.0 - Smaily for OpenCart 1.4.0
2.3.0.2 - Smaily for OpenCart 1.4.0
3.0.3.1 - Smaily for OpenCart 1.4.0