Fail with meaningful error in case an unsupported trigger is used

[simoneb]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

Although the documentation shows code examples using pull_request as a trigger, we have received several reports over time of people trying to use this action with other targets, such as pull_request_target. The motivations are described in #355.

To avoid further reports, we should fail with a meaningful error message when an unexpected trigger is used.

Motivation

No response

Example

No response

[simoneb]

I'm closing this as there is no consensus about what we should do with this conversation.