Cloudwatch: Fix Unexpected error (#74420)

Fix unexpected error when creating a new cloudwatch datasource.

Involves a fair amount of refactoring, so if this causes unexpected issues related to region fetching we can turn this off with the cloudwatchNewRegionsHandler feature toggle, although we do not predict it will so we are enabling it to default to true and hope to remove it shortly.

pkg/tsdb/cloudwatch/models/errors.go

@@ -0,0 +1,7 @@
+package models
+
+import "fmt"
+
+// put misc expected user errors here
+
+var ErrMissingRegion = fmt.Errorf("missing default region")