Throw somewhat better exception if aws-cdk is missing

[lakkeger]

Motivation

In case of aws-cdk is installed by a different package manager (ie brew on Mac OS) as a separate CLI tool cdklocal complains that the aws-cdk module is missing. To resolve this situation I found that the easiest solution is to add it by the NODE_PATH variable that expands the lookup path for modules. (docs)

If the NODE_PATH environment variable is set to a colon-delimited list of absolute paths, then Node.js will search those paths for modules if they are not found elsewhere.

Reported issue: #90

Changes

• update README
• add small message to exception

[dominikschubert]

Thanks for improving the documentation/error reporting on failed imports. Had a few nits but nothing major blocking a merge.