refactor!: generate per/service base exception types

[aajtodd]

Issue #, if available:
closes #233

Description of changes:

• (refactor): generate an exception type named after the service that all other modeled exceptions in the service closure inherit from when generated (e.g. S3Exception). Previously all exceptions inherited directly from AwsServiceException/ServiceException)
• breaking (refactor): changed the logic used to derive a client name from sdkId to respect words that are already pascal case.
  • We had discussed this previously that it would be better to generate ApiGatewayManagementHub instead of Apigatewaymanagementhub for instance (not a real example necessarily). This wasn't strictly necessary but it seemed better to get this in now than later. The base exception types also inherit this naming scheme so we ended up with exceptions like DynamodbException or ElasticacheException instead of DynamoDbException/ElastiCacheException etc.
  • We (ahem) borrowed the Java v2 SDK logic here for how they name their clients

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[kggilmer]

LGTM