Core: ngx_regex_compile() error handling fixes.

Now we actually return NGX_ERROR on errors, and provide an error
string for memory allocation errors.

Reported by Markus Linnala.
Fixed ETag memory allocation error handling.

The etag->hash must be set to 0 to avoid an empty ETag header being
returned with the 500 Internal Server Error page after the memory
allocation failure.

Reported by Markus Linnala.
Added ngx_init_setproctitle() return code check.

The ngx_init_setproctitle() function, as used on systems without
setproctitle(3), may fail due to memory allocation errors, and
therefore its return code needs to be checked.

Reported by Markus Linnala.