Fix base exception message

[wbingli]

Issue #, if available: #243

Description of changes:. Fix the error message issue for BaseHandlerException.

Cloudformation service doesn't like empty message exception, use the cause exception as default exception message.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[miparnisari]

So this is why this code

try {
            awsResponse = proxyClient.injectCredentialsAndInvokeV2(awsRequest, proxyClient.client()::deleteMetricFilter);
        } catch (ResourceNotFoundException e) {
            logger.log("Resource does not exist and could not be deleted.");
            throw new CfnNotFoundException(e);
        }

causes this to be printed in the handler logs...

  | 2020-05-15T15:00:53.943-07:00 | Resource does not exist and could not be deleted.
  | 2020-05-15T15:00:54.011-07:00 | null in a DELETE action on a AWS::Logs::MetricFilter: 
software.amazon.cloudformation.exceptions.CfnNotFoundExceptionsoftware.amazon.cloudformation.exceptions.CfnNotFoundException	
at software.amazon.logs.metricfilter.DeleteHandler.deleteResource(DeleteHandler.java:51)

[aygold92]

I don't see any reason to limit all of these exceptions to just having a cause constructor. Can we include a (cause) and a (message, cause) for all of them?

[wbingli]

Good suggestion. However, I'm afraid it will be other large change to add all exception to support this new constructor as well as make the BaseHandlerException correct.

I would prefer to add this new constructor as new feature, but this PR will focus to fix this known bug.

[aygold92]

can we create an issue to resolve this then, if we're not going to include it in this PR?

[wbingli]

Tracked in #276