[GTK] Add WebKitError to GTK+ WebKit2 API

[GTK] Add WebKitError to GTK+ WebKit2 API
https://bugs.webkit.org/show_bug.cgi?id=68508

Reviewed by Martin Robinson.

Source/WebKit2:

Add WebKitNetworkError enum to define network errors.

* GNUmakefile.am: Add new files to compilation.
* UIProcess/API/gtk/WebKitError.cpp: Added.
(webkit_network_error_quark):
* UIProcess/API/gtk/WebKitError.h:
* UIProcess/API/gtk/WebKitPrivate.h: Add
COMPILE_ASSERT_MATCHING_ENUM macro.
* UIProcess/API/gtk/webkit2.h: Add WebKitError.h and
WebKitEnumTypes.h

Tools:

* GNUmakefile.am: Add path to webkitgtk derived sources to the
include path.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@96225 268f45cc-cd09-0410-ab3c-d52691b4dbfc