Describe the bug
Briefly, describe what broke and provide the following details:

• Operating system

  macOS 15.5
  

• fly version

  fly v0.3.125 darwin/arm64 Commit: c78d4555c62f257c686bd6c0280f1fb1c59a4e4f BuildDate: 2025-05-17T14:55:45Z
  

Command output

When running fly launch with an existing config passed in via the --config <path/to/fly.toml> option, if the file doesn't exist, it fails silently and acts as if no file was provided.

$ fly launch \
  --config non-existent-file.toml \
  --copy-config \
  --ha=false
Scanning source code
Could not find a Dockerfile, nor detect a runtime or framework from source code. Continuing with a blank app.
Creating app in /Users/user/test
We're about to launch your app on Fly.io. Here's what you're getting:
...

The problem was found when passing in a URL for the launch config, which doesn't appear to be supported, but also happens for any not-found file ☝ :

$ fly launch \
  --config https://github.com/stellar/quickstart/raw/refs/heads/flyio/.fly/fly.toml \
  --copy-config \
  --ha=false