readelf: Print attribute name and form in error message.

readelf: Print attribute name and form in error message.

Now an error getting the attribute value will not only print the DIE offset
and the reason, but also the attribute name and form. e.g.

  DIE [b] cannot get attribute 'ranges' (sec_offset) value: .debug_ranges
  section missing

Also we don't abort, but try to print the other attributes of the DIE
anyway. It might just be one attribute whose value cannot be resolved.

Signed-off-by: Mark Wielaard <mark@klomp.org>