fix(core): improve context providers error message for env-agnostic stacks (#3137)

Following up on #2922: improve the error message emitted when trying to read context from an environment-agnostic stack by calling out CDK_DEFAULT_ACCOUNT and CDK_DEFAULT_REGION as a way to inherit environment information from the CLI.

Fixes #3078
Fixes #3120
Fixes #3130

Author: Elad Ben-Israel

packages/@aws-cdk/core/lib/context-provider.ts

@@ -71,7 +71,10 @@ export class ContextProvider {
     const stack = Stack.of(scope);
 
     if (Token.isUnresolved(stack.account) || Token.isUnresolved(stack.region)) {
-      throw new Error(`Cannot retrieve value from context provider ${options.provider} since account/region are not specified at the stack level`);
+      throw new Error(`Cannot retrieve value from context provider ${options.provider} since account/region ` +
+                      `are not specified at the stack level. Either configure "env" with explicit account and region when ` +
+                      `you define your stack, or use the environment variables "CDK_DEFAULT_ACCOUNT" and "CDK_DEFAULT_REGION" ` +
+                      `to inherit environment information from the CLI (not recommended for production stacks)`);
     }
 
     const { key, props } = this.getKey(scope, options);