crypto/openpgp: add error and armor

crypto/openpgp: add error and armor

error is needed by all the OpenPGP packages as they return a shared
family of error types.

armor implements OpenPGP armoring. It's very like PEM except:
  a) it includes a CRC24 checksum
  b) PEM values are small (a few KB) and so encoding/pem assumes that
     they fit in memory. Armored data can be very large and so this
     package presents a streaming interface.

R=r, nsz, rsc
CC=golang-dev
https://golang.org/cl/3786043