[feature] Ability to define custom error notification text

**Version and OS**
v0.52.9 on linux/k8

**Is your feature request related to a problem? Please describe.**
Currently when a monitor encounters an error, a default error text is sent as notification. It breaks the JSON endpoints.

**Describe the solution you'd like**
Ability to specify error notification text just like other notification text we can specify in the global settings.
