feat(region-info): throw exception when no fact found (#5166)

* New method that throws an exception when the Fact value, given a name and region, are not found.

closes #3194

packages/@aws-cdk/region-info/lib/fact.ts

@@ -23,6 +23,23 @@ export class Fact {
     return regionFacts && regionFacts[name];
   }
 
+  /**
+   * Retrieve a fact from the Fact database. (retrieval will fail if the specified region or
+   * fact name does not exist.)
+   *
+   * @param region the name of the region (e.g: `us-east-1`)
+   * @param name the name of the fact being looked up (see the `FactName` class for details)
+   */
+  public static requireFact(region: string, name: string): string {
+    const foundFact = this.find(region, name);
+
+    if (!foundFact) {
+      throw new Error(`No fact ${name} could be found for region: ${region} and name: ${name}`);
+    }
+
+    return foundFact;
+  }
+
   /**
    * Registers a new fact in this Fact database.
    *

packages/@aws-cdk/region-info/test/fact.test.ts

@@ -11,6 +11,16 @@ describe('find', () => {
   });
 });
 
+describe('requireFact', () => {
+  test('throws error for an unknown fact', () => {
+    expect(() => Fact.requireFact(AWS_REGIONS[0], 'not:a:known:fact')).toThrowError();
+  });
+
+  test('throws error for an unknown region', () => {
+    expect(() => Fact.requireFact('bermuda-triangle-42', FactName.PARTITION)).toThrowError();
+  });
+});
+
 describe('register', () => {
   test('allows registering an arbitrary fact', () => {
     // GIVEN