proto, internal/errors: add Error sentinel, errors.Wrap

Add a sentinel proto.Error error which matches all errors returned by
packages in this module.

Document that protoregistry.NotFound is an exact sentinel value for
performance reasons.

Add a Wrap function to the internal/errors package and use it to wrap
errors from outside sources (resolvers). Wrapped errors match
proto.Error.

Fixes golang/protobuf#1021.

Change-Id: I45567df3fd6c8dc9a5caafdb55654827f6fb1941
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/215338
Reviewed-by: Joe Tsai <joetsai@google.com>
diff --git a/reflect/protodesc/desc_resolve.go b/reflect/protodesc/desc_resolve.go
index 1bf4799..cebb36c 100644
--- a/reflect/protodesc/desc_resolve.go
+++ b/reflect/protodesc/desc_resolve.go
@@ -187,7 +187,7 @@
 			}
 			foundButNotImported = d
 		} else if err != protoregistry.NotFound {
-			return nil, err
+			return nil, errors.Wrap(err, "%q", s)
 		}
 
 		// Continue on at a higher level of scoping.