Catch and handle errors of FCM

[MizukiTemma]

Short description

This PR add an error handling for invalid FCM URL.

update This takes care of timeout exception too. I've totally forgotten that those two issues are related each other.

Proposed changes

• Catch errors and show an error message to users
• The error message is left universal (not specific to any error type) because many types are possible.

Side effects

• The rendering after failing of a push notification is partially broken.
• A question regarding this rendering: don't we want to redirect not no the create view but to the edit view becuase the push notification is saved in this case?
• I'll suggest to fix the rendering in a separate PR/issue.

Resolved issues

Fixes: #2025 , #2790

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