RFR: JDK-8234687: change javap reporting on unknown attributes

[Jonathan Gibbons]

Please review a small change to javap, to avoid reporting an unknown or 
custom attribute as an "error".

The following is an example of the javap output, before and after the 
change:

Before:

Error: unknown attribute
   CustomAttr: length = 0x2
    00 0E

After:

   CustomAttr: length = 0x2 (unknown attribute)
    00 0E


The test is a modified copy of an existing test, despite appearing in 
the webrev to be a modified renamed test.  It shares enough in common 
with the other test to be worth sharing its genesis.

-- Jon


JBS: https://bugs.openjdk.java.net/browse/JDK-8234687
Webrev: http://cr.openjdk.java.net/~jjg/8234687/webrev.00/