generate server-side exceptions as classes

[AllanZhengYP]

Issue #, if available:
Related to aws/aws-sdk-js-v3#3261
SSDK generated example: aws/aws-sdk-js-v3#3310

Description of changes:
This change generates server-side exceptions as individual classes. So customers can handle exceptions with ease.
You can view generated code diff at aws/aws-sdk-js-v3#3267

Now the customer can handle the errors without hardcoding the error code:

import {
    InvalidSignatureException,
    UnfoundResourceException,
    FooServiceException
} from "@aws-sdk/client-foo";

try {
    await client.send(operation);
} catch (e) {
    if (e instanceof InvalidSignatureException) {
    // Do something
    } else if (e instanceof UnfoundResourceException) {
    // Do other thing
    } else if (e instanceof FooServiceException) {
    // Unknown exception
    } else {
    // SDK error
    }
}

For SSDK, customer can do the similar:

import { InvalidSignatureException, UnfoundResourceException } from "@aws-sdk/server-foo";
try {
    await operation.handle(...);
} catch (e) {
    if (e instanceof InvalidSignatureException) {
    // Do something
    } else if (e instanceof UnfoundResourceException) {
    // Do other thing
    } else {
    // SDK error
    }
}

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[gosar]

The commit for the generated code linked in the PR description says SdkException. Probably outdated link. It would be with ServiceException, right?

[AllanZhengYP]

@gosar Yes, they will be renamed to ServiceExceptions. Updated the description.

[gosar]

Please update the link in the description of the PR with the latest generated code