caddyhttp: Always log handled errors at debug level

[francislavoie]

Context: https://caddy.community/t/optional-reverse-proxy-with-file-system-fallback/15105

There are some situations where error handling is used to intercept 5xx errors emitted by Caddy, like from reverse_proxy, and those errors would still be logged at the ERROR level.

This could get noisy when this is being done "on-purpose". For example, a proxy being turned off half the time and only used as an optimization during development (for hot-module reload or something).

So instead, it probably makes more sense to always log handled errors at DEBUG level, since it was actually handled. If the error falls through unhandled, or a new error is triggered, then it'll still log at ERROR level.

[mholt]

LGTM, and makes sense. User is already handling the error, so no need to log it as an error (unless their error handling also has an error).

[delfick]

super cool. Thanks!