Update Codebuild event to include CLIENT_ERROR as a PhaseStatus type

**Is your feature request related to a problem? Please describe.**

My team has encountered a `VPC_CLIENT_ERROR: Unexpected EC2 error: RequestLimitExceeded` error when load testing CodeBuild.

Inspecting the build details in the web console, the status appears as a client error:

However, when using the [codebuild event](https://github.com/aws/aws-lambda-go/blob/master/events/codebuild.go#L17) to unmarshall the event, we are receiving an empty string for the phase's status.

**Describe the solution you'd like**
I'd like to be able to know when a client error occurs. This could be supported by
adding the CLIENT_ERROR as a supported Phase Status type like:

```
CodeBuildPhaseStatusClientError CodeBuildPhaseStatus = "CLIENT_ERROR"
```