all: improve panic messages for better debugability

Change-Id: If3e505e715d5ce2c9a81249c868d26226a25b724
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/232339
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/impl/message_reflect_gen.go b/internal/impl/message_reflect_gen.go
index 7d65f5c..741d6e5 100644
--- a/internal/impl/message_reflect_gen.go
+++ b/internal/impl/message_reflect_gen.go
@@ -114,7 +114,7 @@
 	if oi := m.messageInfo().oneofs[od.Name()]; oi != nil && oi.oneofDesc == od {
 		return od.Fields().ByNumber(oi.which(m.pointer()))
 	}
-	panic("invalid oneof descriptor")
+	panic("invalid oneof descriptor " + string(od.FullName()) + " for message " + string(m.Descriptor().FullName()))
 }
 func (m *messageState) GetUnknown() protoreflect.RawFields {
 	m.messageInfo().init()
@@ -234,7 +234,7 @@
 	if oi := m.messageInfo().oneofs[od.Name()]; oi != nil && oi.oneofDesc == od {
 		return od.Fields().ByNumber(oi.which(m.pointer()))
 	}
-	panic("invalid oneof descriptor")
+	panic("invalid oneof descriptor " + string(od.FullName()) + " for message " + string(m.Descriptor().FullName()))
 }
 func (m *messageReflectWrapper) GetUnknown() protoreflect.RawFields {
 	m.messageInfo().init()