[errors] de-specialize Transient in favor of Tags.

common/config/validation/validation.go

Index: common/config/validation/validation.go
diff --git a/common/config/validation/validation.go b/common/config/validation/validation.go
index c5e8d433684eb56edcbd653f4e98ae479414af40..b7807211c2c01c1945899b95dd4c642fd9fb269c 100644
--- a/common/config/validation/validation.go
+++ b/common/config/validation/validation.go
@@ -44,6 +44,9 @@ type Context struct {
 	element []string          // logical path of a sub-element we validate, see Enter
 }
 
+var fileTag = errors.NewTagger("holds the file name for tests")
+var element0Tag = errors.NewTagger("holds the first element for tests")
+
 // Error records a validation error.
 func (v *Context) Error(msg string, args ...interface{}) {
 	ctx := ""
@@ -52,7 +55,9 @@ func (v *Context) Error(msg string, args ...interface{}) {
 	} else {
 		ctx = "in <unspecified file>"
 	}
+	tags := []errors.TagValue{fileTag.With(v.file)}
 	if len(v.element) != 0 {
+		tags = append(tags, element0Tag.With(v.element[0]))
 		ctx += " (" + strings.Join(v.element, " / ") + ")"
 	}
 
@@ -66,9 +71,7 @@ func (v *Context) Error(msg string, args ...interface{}) {
 	}
 
 	// Make the file and the logical path also usable through error inspection.
-	err := errors.Reason(fmt.Sprintf(msg, args...)).
-		D("file", v.file).
-		D("element", append([]string{}, v.element...)).Err()
+	err := errors.Reason(fmt.Sprintf(msg, args...)).Tag(tags...).Err()
 	v.errors = append(v.errors, err)
 }