GeezerVison

The resource docs do not explain how a processor can refuse to write data and display an error to the user.

This adds an "Example #3: Prevent an operation and display an error" section to docs/resource/processors.md:

• a delete processor throwing a WriteResourceException (an accepted subscription cannot be deleted);
• the order in which the error flash translation key is looked up in the flashes domain, and the %resource% parameter;
• a warning that the first constructor argument of WriteResourceException is the resource name, so the key goes in the named message argument.

The first lookup step (the message of the exception) depends on Sylius/SyliusResourceBundle#1172, which targets 1.15 — this PR should be merged after it. Without it, the rest of the section already describes the current behaviour.

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