configure.ac: no longer test version-sensitive libvala, instead use AM_PROG_VALAC.  Fix bgo #706521.
Change error handling from throwing errors to setting a document-level error variable (2)
Remove all throws declarations, since we're shifting to a document-level error variable
Change some errors in CharacterData and Text to use g_warning instead of GError
Document.vala: Update comments, particularly with references to spec
Document.vala: update error checking to use g_warning; add more error checks from spec (2)
CharacterData.vala: update comments, particularly adding references to spec
CharacterData.vala: improve error handling, check more bounds, factor out bound checking
Text.vala: make use of new check_index_size from parent class CharacterData
Document.vala, DomNode.vala: move check_read_only to DomNode, so all its descendents can call it
Document.vala, DomNode.vala: move check_wrong_document inot DomNode so other nodes (Element) can use it
Element.vala: handle spec'd errors
ProcessingInstruction.vala: handle spec'd errors
Text.vala: add more error checking from spec
Tiny fixes.

DomNode.vala: make check_wrong_document and check_read_only protected so only subclasses can see them
Element.vala: fix remove_attribute_node
ProcessingInstruction: properly get and set 'data'
Document.vala: some small error handling changes; return on invalid chars for create_element
Serialization.vala: DomError no longer thrown, so no need to catch it
GXmlTest.vala: override gtester handling of g_warnings; don't want to abort, but verify
DomException.vala: change errordomain DomError -> enum DomException
DomException.vala: add a dummy DomError for the transitioning of the tests