Retain backtrace for TypeErrors in value change callback (#584)

This code was originally added to provide better error messaging
for TypeErrors when assigning values.

Unfortunately throwing a new error object causes us to lose
the context (including stacktrace) of the original error,
making it harder to locate the source.

Changing the message and rethrowing the original error
retain that information.

src/core/value_observer.ts

@@ -96,9 +96,11 @@ export class ValueObserver implements StringMapObserverDelegate {
 
         changedMethod.call(this.receiver, value, oldValue)
       } catch (error) {
-        if (!(error instanceof TypeError)) throw error
+        if (error instanceof TypeError) {
+          error.message = `Stimulus Value "${this.context.identifier}.${descriptor.name}" - ${error.message}`
+        }
 
-        throw new TypeError(`Stimulus Value "${this.context.identifier}.${descriptor.name}" - ${error.message}`)
+        throw error
       }
     }
   }