better error messages when a property namespace isn't registered

[lsmith77]

jackalope itself should check for valid namespace prefixes.

jackalope-doctrine-dbal fails during save with

Notice: Undefined index: phpcr_locale in .../jackalope-doctrine-dbal/src/Jackalope/Transport/DoctrineDBAL/Client.php line 440

jackalope-jackrabbit fails on save too, while talking to the backend

Sending this message without having "phpcr" registered as a namespace:
string(625) "<sv:node xmlns:sv="http://www.jcp.org/jcr/sv/1.0" xmlns:nt="http://www.jcp.org/jcr/nt/1.0" sv:name="user"><sv:property sv:name="jcr:primaryType" sv:type="Name">sv:valuent:unstructured/sv:value/sv:property<sv:property sv:name="username" sv:type="String">sv:value/sv:value/sv:property<sv:property sv:name="numbers" sv:type="Long">sv:value3/sv:valuesv:value1/sv:valuesv:value2/sv:value/sv:property<sv:property sv:name="phpcr:class" sv:type="String">sv:value/sv:value/sv:property/sv:node"

Leads to this is the response:
string(317) "<D:error xmlns:D="DAV:"><dcr:exception xmlns:dcr="http://www.day.com/jcr/webdav/1.0">dcr:classjavax.jcr.InvalidSerializedDataException/dcr:classdcr:messageillegal property name: {http://www.jcp.org/jcr/sv/1.0}property/dcr:message/dcr:exception/D:error"

And finally this exception:
PHPCR\InvalidSerializedDataException: HTTP 400: illegal property name: {http://www.jcp.org/jcr/sv/1.0}property

[dbu]

looks like we have to check the namespaces on client side, i guess jackrabbit does on client side too and the server does not expect this error just says f... you when we use an unregistered namespace. would be good anyways if the transport does not have to validate namespaces.