Reviewed by Darin Adler.

Errors encountered within SVG documents should be reported to the console
https://bugs.webkit.org/show_bug.cgi?id=62599
<rdar://problem/9727074>

Add SVGLength::construct, which can be used to construct an SVGLength
as normal, but returns an SVGParsingError via a reference argument.
The function also takes another argument, which is whether or not
the SVGLength value is allowed to be negative. If not, an error is
thrown. It defaults to allowing negatives if not specified.

* GNUmakefile.list.am:
* svg/SVGLength.cpp:
(WebCore::SVGLength::construct):
(WebCore::SVGLength::rawValue):
* svg/SVGLength.h:
* svg/SVGElement.h:
* svg/SVGParsingError.h: Added.
* WebCore.gypi:
* WebCore.pro:
* WebCore.vcproj/WebCore.vcproj:
* WebCore.xcodeproj/project.pbxproj:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@91437 268f45cc-cd09-0410-ab3c-d52691b4dbfc