Add variants of Get functions with existence checks

Currently, a translation that doesn't exist just defaults to the
passed message ID.

It can be helpful to be able to catch these missing cases e.g. to
save to a log.

This PR implements variants ending in 'E' like GetE, GetNE.

This wasn't implemented at the package-level scope, because for
quick translations like that the extra flexibility probably isn't
needed.

translator.go

@@ -13,11 +13,17 @@ import "net/textproto"
 type Translator interface {
 	ParseFile(f string)
 	Parse(buf []byte)
+
 	Get(str string, vars ...interface{}) string
 	GetN(str, plural string, n int, vars ...interface{}) string
 	GetC(str, ctx string, vars ...interface{}) string
 	GetNC(str, plural string, n int, ctx string, vars ...interface{}) string
 
+	GetE(str string, vars ...interface{}) (string, bool)
+	GetNE(str, plural string, n int, vars ...interface{}) (string, bool)
+	GetCE(str, ctx string, vars ...interface{}) (string, bool)
+	GetNCE(str, plural string, n int, ctx string, vars ...interface{}) (string, bool)
+
 	MarshalBinary() ([]byte, error)
 	UnmarshalBinary([]byte) error
 }