dtlib: (cosmetic) use filename and line number on all messages

Take advantage of the new 'filename' and 'lineno' attributes of Node and
Property objects to provide more informative error and status messages
by printing the actual source file reference of the corresponding
object, always using the unquoted file:line format in error messages.

No functional change is introduced by this commit.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>