Improve logging and exception handling for UILeaf

If #{} (without a component tag) throws a exception (mainly in render response), it is practically impossible for user to find out which element was the source.

Suggestions:

• use Location and TagAttributeAware EL context wrappers from MYFACES-3202 in ELText
• put UIComponent.VIEW_LOCATION_KEY into UILeaf during VDL.buildView
• improve toString for UIInstructions (output sometimes spans over three or more lines)