config: clarify unknown variable error [GH-1480]

CHANGELOG.md

@@ -3,6 +3,7 @@
 BUG FIXES:
 
   * core: refresh won't remove outputs from state file [GH-1369]
+  * core: clarify "unknown variable" error [GH-1480]
   * provider/aws: fix panic possibility if RDS DB name is empty [GH-1460]
   * provider/aws: fix issue detecting credentials for some resources [GH-1470]
   * provider/google: fix issue causing unresolvable diffs when using legacy

config/config.go

@@ -207,7 +207,7 @@ func (c *Config) Validate() error {
 
 			if _, ok := varMap[uv.Name]; !ok {
 				errs = append(errs, fmt.Errorf(
-					"%s: unknown variable referenced: %s",
+					"%s: unknown variable referenced: '%s'. define it with 'variable' blocks",
 					source,
 					uv.Name))
 			}