Handle backend rate limits gracefully

[ChristophWurst]

Steps to reproduce

1. Use this app with fix(dav): Rate limit calendar/subscription creation server#43732
2. Create too many calendars too fast

Expected behavior

Human readable error explaining the situation

Actual behaviour

Generic error that doesn't say much

Calendar app version

No response

CalDAV-clients used

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Browser

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Client operating system

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Server operating system

No response

Web server

None

Database engine version

None

PHP engine version

None

Nextcloud version

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Updated from an older installed version or fresh install

None

List of activated apps

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Nextcloud configuration

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Web server error log

No response

Log file

No response

Browser log

No response

Additional info

No response