Bug fix: catch errors from UserDataBuilder.AddFile

[PettitWesley]

Fixes a minor bug: previously errors weren't caught on from the AddFile method for user data. This meant that something like this will erroneously succeed:

ecs-cli up --capability-iam --extra-user-data some_file_that_doesnt_exit

This PR fixes the issue by checking the error.

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

[SoManyHs]

Could you add manual test output from before and after?

[PettitWesley]

@SoManyHs

Before (command erroneously succeeds):

$ ecs-cli up --cluster oops --region sa-east-1 --capability-iam --extra-user-data some_file_that_does_not_exist
WARN[0000] You will not be able to SSH into your EC2 instances without a key pair. 
INFO[0002] Using recommended Amazon Linux AMI with ECS Agent 1.21.0 and Docker version 18.06.1-ce 
INFO[0003] Created cluster                               cluster=oops region=sa-east-1
INFO[0005] Waiting for your cluster resources to be created... 
INFO[0005] Cloudformation stack status                   stackStatus=CREATE_IN_PROGRESS
INFO[0068] Cloudformation stack status                   stackStatus=CREATE_IN_PROGRESS
INFO[0130] Cloudformation stack status                   stackStatus=CREATE_IN_PROGRESS

After:

$ ecs-cli up --cluster oops --region sa-east-1 --capability-iam --extra-user-data some_file_that_does_not_exist
WARN[0000] You will not be able to SSH into your EC2 instances without a key pair. 
FATA[0000] Error executing 'up': open some_file_that_does_not_exist: no such file or directory