aws-tools-env.sh: Allow AWS_CREDENTIAL_FILE and ELASTIC_MAPREDUCE_CREDENTIALS to be preset.

This is useful if there are multiple processes that want to use different
credentials. They can each preset AWS_CREDENTIAL_FILE to a unique location.
aws-tools-env.sh: Use || instead of $? check.

This keeps the script from failing if it's sourced into a
shell that is running with -e.