Prevent fetch calls on IDL files

[peats-bond]

This change prevents users from calling idl fetch on a thrift file. Users should be calling fetch with a service name instead.

A reported issue discovered that update wasn't fetching the latest IDL for a service. Investigation showed that meta.json had incorrect entries: some remotes specified a .thrift extension. This indicates users calling idl fetch on a specifc IDL file. Reading the meta.json file on later calls to fetch and update caused IDL to silently fail with a 0 exit code.

Users would see IDL updates up to the first invalid entry and then IDL would quit.

This change will update entries with .thrift extensions for backwards compatibility.

[CLAassistant]

All committers have signed the CLA.