scripts/dts: Remove block in extract_property()

Turn

  if a:
    if b:
      ...

into

  if a and b:
    ...

Simplify some error messages with .format() as well, and get rid of some
redundant str()s. '{0} {1} {1}'.format('foo', 'bar') gives
'foo bar bar'.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>