fix(cli): turn validation errors into metadata errors

Validation errors are currently turned into exceptions. This causes
issues where missing context values lead to validation errors (such as
in the case of `DnsValidatedCertificate`): the error would have been
solved by retrieving the context value, but because an exception is
thrown the CLI stops immediately and doesn't re-execute.

I feel it also makes sense to treat these errors as construct errors,
as opposed to exceptions which I feel should be more "you're misusing
the API" kind of errors, rather than "something is wrong somewhere"
kind of errors.

The change I made is the smallest change to achieve the desired effect.
If we agree that we want this change, it would probably be better to
do it differently, just have `.validate()` implementors call
`this.node.addError()` immediately.

Fixes #2076.

packages/@aws-cdk/cdk/lib/construct.ts

@@ -48,10 +48,11 @@ export class ConstructNode {
       const validate = options.skipValidation === undefined ? true : !options.skipValidation;
       if (validate) {
         const errors = this.validate(root);
-        if (errors.length > 0) {
-          const errorList = errors.map(e => `[${e.source.node.path}] ${e.message}`).join('\n  ');
-          throw new Error(`Validation failed with the following errors:\n  ${errorList}`);
-        }
+        // Turn the errors into metadata errors, so they will prevent deploying but will not
+        // prevent CDK from executing again.
+        errors.forEach(e => {
+          e.source.node.addError(e.message);
+        });
       }
 
       // synthesize (leaves first)

packages/@aws-cdk/cdk/test/test.app.ts

@@ -156,7 +156,11 @@ export = {
   new Child(parent, 'C1');
   new Child(parent, 'C2');
 
-  test.throws(() => app.synth(), /Validation failed with the following errors/);
+  const assembly = app.synth();
+  test.deepEqual(assembly.getStack('Parent').messages.filter(m => m.level === cxapi.SynthesisMessageLevel.ERROR).map(m => m.entry.data), [
+    'Error from C1',
+    'Error from C2',
+  ]);
 
   test.done();
 },