all: rename IsInitialized as CheckInitialized

An Is prefix implies it returns a boolean.
A Check prefix better suggests that it could return an error.

Change-Id: I6ffcb32099a944c656c07654c294a0980efb2d0e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/220338
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/impl/codec_message.go b/internal/impl/codec_message.go
index 08f4d5a..a86468a 100644
--- a/internal/impl/codec_message.go
+++ b/internal/impl/codec_message.go
@@ -148,8 +148,8 @@
 		mi.methods.Flags |= piface.SupportUnmarshalDiscardUnknown
 		mi.methods.Unmarshal = mi.unmarshal
 	}
-	if mi.methods.IsInitialized == nil {
-		mi.methods.IsInitialized = mi.isInitialized
+	if mi.methods.CheckInitialized == nil {
+		mi.methods.CheckInitialized = mi.checkInitialized
 	}
 	if mi.methods.Merge == nil {
 		mi.methods.Merge = mi.merge