Fix bug with error files (e.g. 404.html) introduced in commit 90de10cf

Deuce expanded the local 'error_code' buf to not trip the SAFECOPY macro
destionation buffer length check. Only this buffer needed to be 4 chars
exactly to truncate the 'message' string at 3 chars (e.g. just "404" for an
error message string that starts with "404 "). Switch from SAFECOPY to strlcpy
to eliminate this bug.

As reported by Ragnarok as part of issue #748

The web error files should work again.