Normalize namespaces and clean up exception handling.

[johnkenny54]

• force elements in the SVG namespace to have no prefix (current architecture does not handle namespaces and prefixes).
• force force attributes in the xlink namespace to have xlink: prefix (many plugins are hard-coded to this prefix)
• throw an Error if non-svg default namespace is encountered outside of a foreignObject
• change optimize() to not throw an Error if there is a parse error - warning message is printed, input string is returned unchanged, and Error is added to return object