fix: Improve error message in SSMParameterProvider (#1630)

When using a non-existent SSM Parameter with the SSMParameterProvider,
the error message `null` was returned by the toolkit instead of a more
meaningful error mentioning important details such as the parameter name
that was looked up and the account/region where it could not be found.

Fixes #1621

packages/aws-cdk/lib/context-providers/ssm-parameters.ts

@@ -1,3 +1,4 @@
+import AWS = require('aws-sdk');
 import { Mode, SDK } from '../api';
 import { debug } from '../logging';
 import { ContextProviderPlugin } from './provider';
@@ -18,11 +19,32 @@ export class SSMContextProviderPlugin implements ContextProviderPlugin {
     const parameterName = args.parameterName;
     debug(`Reading SSM parameter ${account}:${region}:${parameterName}`);
 
-    const ssm = await this.aws.ssm(account, region, Mode.ForReading);
-    const response = await ssm.getParameter({ Name: parameterName }).promise();
+    const response = await this.getSsmParameterValue(account, region, parameterName);
     if (!response.Parameter || response.Parameter.Value === undefined) {
       throw new Error(`SSM parameter not available in account ${account}, region ${region}: ${parameterName}`);
     }
     return response.Parameter.Value;
   }
+
+  /**
+   * Gets the value of an SSM Parameter, while not throwin if the parameter does not exist.
+   * @param account       the account in which the SSM Parameter is expected to be.
+   * @param region        the region in which the SSM Parameter is expected to be.
+   * @param parameterName the name of the SSM Parameter
+   *
+   * @returns the result of the ``GetParameter`` operation.
+   *
+   * @throws Error if a service error (other than ``ParameterNotFound``) occurs.
+   */
+  private async getSsmParameterValue(account: string, region: string, parameterName: string): Promise<AWS.SSM.GetParameterResult> {
+    const ssm = await this.aws.ssm(account, region, Mode.ForReading);
+    try {
+      return await ssm.getParameter({ Name: parameterName }).promise();
+    } catch (e) {
+      if (e.code === 'ParameterNotFound') {
+        return {};
+      }
+      throw e;
+    }
+  }
 }