specerror: Add FilterError helper

This doesn't save much time for folks who are iterating over the
removed errors (e.g. to print warnings about them), but Aleksa asked
for it [1].

I haven't used it in runtimetest.  I'm waiting for the in-flight
7aa3987 (cmd/runtimetest/main: Use TAP diagnostics for errors,
2017-07-28, opencontainers#439) to settle, since that commit cleans up some of the
runtimetest error-processing code with:

  validations := defaultValidations
  if platform == "linux" {
    validations = append(validations, linuxValidations...)
  }

and a single loop over the constructed validations array.

I initially had FilterError returning (filtered, removed), but golint
didn't like that:

  error should be the last type when returning multiple items

[1]: opencontainers#492 (comment)

Signed-off-by: W. Trevor King <wking@tremily.us>

cmd/oci-runtime-tool/validate.go

@@ -3,7 +3,6 @@ package main
 import (
 	"fmt"
 
-	"github.com/hashicorp/go-multierror"
 	rfc2119 "github.com/opencontainers/runtime-tools/error"
 	"github.com/opencontainers/runtime-tools/specerror"
 	"github.com/opencontainers/runtime-tools/validate"
@@ -37,20 +36,13 @@ var bundleValidateCommand = cli.Command{
 		}
 
 		if err := v.CheckAll(); err != nil {
-			merr, ok := err.(*multierror.Error)
-			if !ok {
-				return err
-			}
-			var validationErrors error
-			for _, err = range merr.Errors {
-				e, ok := err.(*specerror.Error)
-				if ok && e.Err.Level < complianceLevel {
+			err, removed := specerror.FilterError(err, complianceLevel)
+			if removed != nil {
+				for _, e := range removed.Errors {
 					logrus.Warn(e)
-					continue
 				}
-				validationErrors = multierror.Append(validationErrors, err)
 			}
-			return validationErrors
+			return err
 		}
 		fmt.Println("Bundle validation succeeded.")
 		return nil

specerror/error.go

@@ -168,3 +168,22 @@ func FindError(err error, code Code) Code {
 	}
 	return NonRFCError
 }
+
+// FilterError removes RFC 2119 errors with a level less than 'level'
+// from the source error.  If the source error is not a multierror, it
+// is returned unchanged.
+func FilterError(err error, level rfc2119.Level) (filtered error, removed *multierror.Error) {
+	merr, ok := err.(*multierror.Error)
+	if !ok {
+		return err, nil
+	}
+	for _, err = range merr.Errors {
+		e, ok := err.(*Error)
+		if ok && e.Err.Level < level {
+			removed = multierror.Append(removed, e)
+			continue
+		}
+		filtered = multierror.Append(filtered, err)
+	}
+	return filtered, removed
+}