Change regex for recognizing errors in the syslog format

[rzadp]

I have found that err is used as an alias to error.
Changing the regex to catch err would work for both usages.

[nodiscc]

This would result in a huge number of false positives since many messages that are not errors contain the string err (see also #733).

I suggest not merging this as-is (and instead use overrides like documented in https://docs.lnav.org/en/latest/formats.html, #1113 improves the docs a bit). For me this would result in false positives for all messages containing referral, override overridden, deferred, preferred, interrupt, providerruntime, ownerrole ... Many randomly generated strings/IDs are also likely to contain err.