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
https://golang.org/cl/5305068

src/pkg/crypto/openpgp/error/error.go

@@ -41,21 +41,21 @@ func (b SignatureError) String() string {
 	return "OpenPGP signature invalid: " + string(b)
 }
 
-type keyIncorrect int
+type keyIncorrectError int
 
-func (ki keyIncorrect) String() string {
+func (ki keyIncorrectError) String() string {
 	return "the given key was incorrect"
 }
 
-var KeyIncorrectError = keyIncorrect(0)
+var KeyIncorrectError = keyIncorrectError(0)
 
-type unknownIssuer int
+type unknownIssuerError int
 
-func (unknownIssuer) String() string {
+func (unknownIssuerError) String() string {
 	return "signature make by unknown entity"
 }
 
-var UnknownIssuerError = unknownIssuer(0)
+var UnknownIssuerError = unknownIssuerError(0)
 
 type UnknownPacketTypeError uint8