Seems nobody has stumbled on the correct answer yet, so here it is.
In NSXMLParserError docs, it says:
The following error codes are defined by NSXMLParser. For error codes not listed here, see the <libxml/xmlerror.h> header file.
The number 111 isn't mentioned in this list, so we go to /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/include/libxml2/libxml/xmlerror.h
, and find the value:
XML_ERR_USER_STOP, /* 111 */
There isn't a lot of documentation on XML_ERR_USER_STOP
in libxml2, but from reading the changeset, it looks like it's a fast-fail when the parser sees an unexpected EOF.