image removal: refactor part 2

Continue the refactoring of image removal.  I didn't manage to break all
the following changes into smaller and easier to digest commits due to
time constraints:

 * Return an error slice instead of a single error. Use multierror only
   in the client/frontend.  Reflect that in the types.

 * Use the batch image removal in the client while preserving the more
   rest-idiomatic single-image removal endpoint.

 * Add a new handler for the single-image removal endpoint to make it
   share the same code as the batch endpoint.

 * Expose bindings for the single and batch endpoints, so we can
   properly test them.

 * Add several convenience functions for error handling to
   pkg/errorhandling.

 * Set the correct error type in libpod to set the exit code to 2 when
   one or more containers are using an image.

 * Massage the bindings tests a bit and tackle compilation errors.

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

pkg/errorhandling/errorhandling.go

@@ -2,10 +2,46 @@ package errorhandling
 
 import (
 	"os"
+	"strings"
 
+	"github.com/hashicorp/go-multierror"
+	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 )
 
+// JoinErrors converts the error slice into a single human-readable error.
+func JoinErrors(errs []error) error {
+	if len(errs) == 0 {
+		return nil
+	}
+
+	// `multierror` appends new lines which we need to remove to prevent
+	// blank lines when printing the error.
+	var multiE *multierror.Error
+	multiE = multierror.Append(multiE, errs...)
+	return errors.New(strings.TrimSpace(multiE.ErrorOrNil().Error()))
+}
+
+// ErrorsToString converts the slice of errors into a slice of corresponding
+// error messages.
+func ErrorsToStrings(errs []error) []string {
+	strErrs := make([]string, len(errs))
+	for i := range errs {
+		strErrs[i] = errs[i].Error()
+	}
+	return strErrs
+}
+
+// StringsToErrors converts a slice of error messages into a slice of
+// corresponding errors.
+func StringsToErrors(strErrs []string) []error {
+	errs := make([]error, len(strErrs))
+	for i := range strErrs {
+		errs[i] = errors.New(strErrs[i])
+	}
+	return errs
+}
+
 // SyncQuiet syncs a file and logs any error. Should only be used within
 // a defer.
 func SyncQuiet(f *os.File) {