Selective strict

[robross0606]

If strict is enabled, TAR_ENTRY_INFO messages cause an error which prematurely ends the operation even though it is:

An informative error indicating that an entry is being modified, but otherwise processed normally.

Conversely, disabling strict causes some warnings which we would prefer to abort early (e.g. TAR_ENTRY_INVALID). Is there a good pattern for applying "strictness" only to certain code values? For example, is there a clean way in an onwarn() handler to immediately treat something as strict by throwing a properly formed error?