crypto/openpgp/error: use Error in names of error impl types
crypto/openpgp/error: use Error in names of error impl types

Will make gofix for error work better.
There is no other indication in this file that
these are actually error implementations.

(They are only used elsewhere.)

R=bradfitz
CC=golang-dev
http://codereview.appspot.com/5305068

dsriram/golang,tv42/old-go,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,gnanderson/go,gnanderson/go,tv42/old-go,apparentlymart/golang,tv42/old-go,dsriram/golang,gnanderson/go,dsriram/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme