Return a better error message when dynamic config file cannot be parsed as yaml

[vanstee]

Currently cogctl responds with cogctl: ERROR: "Internal server error" if the yaml file could not be parsed as yaml (in this case an ENV var looking string was uploaded). We should make sure we just respond with cogctl: ERROR: "Config file could not be parsed as yaml".

[christophermaier]

Further context: this was uncovered when a file with the contents

AWS_ACCESS_KEY_ID=XXX
AWS_SECRET_ACCESS_KEY=XXX

was uploaded via cogctl. Instead, it should have been the YAML

---
AWS_ACCESS_KEY_ID: "XXX"
AWS_SECRET_ACCESS_KEY: "XXX"

Somehow, the contents of the first file made it to Cog as a single string, instead of the map that was expected. This caused an Ecto changeset error, which resulted in the aforementioned "Internal server error".