[6.x] Fix share_errors breaking nocache regions on successful responses

[joshuablum]

Fixes a bug where enabling static_caching.share_errors silently breaks {{ nocache }} regions on regular 200 responses when using half measure static caching.
Affected pages log Static cache region [...] not found on [URL]. Serving uncached response. on every request after the first, fall through to a full dynamic render, and lose the benefit of static caching. Funny thing is that the Control Panel still reports them as cached.

The fix is an early return from copyError() when $response->isSuccessful(). The shared error cache is only ever consumed by RendersHttpExceptions for actual 4xx/5xx exceptions, so successful responses have no reason to go through the shared error caching path.
The shared error behavior for genuine error responses is unchanged.

Closes #11700.

Related:

• [5.x] Fix nocache tags on shared error pages #10340
• Static cache region [bac2e48b302...] not found on [https://...]. Serving uncached response. #12216