Add tests to bring coverage to 82.1%

3 more tests (coverage at 82.1%):

• helm hooks
• generator file not existing
• generator file having bad formatting (can't be parsed as yaml)

Updated error processing:

• remove one redundant error return statement
• add one missing error return statement

Error changes ensure that the built binary always returns an accurate error. Examples:

> ./kustomize-helmgenerator filethatdoesntexist
Error: open filethatdoesntexist: no such file or directory
> ./kustomize-helmgenerator bad-yaml.yaml
Error: yaml: line 10: mapping values are not allowed in this context