Return better error messages for missing config

[jonjohnsonjr]

Fixes #1001

$ apko build /does/not/exist  example.com:latest ./out
ℹ️            | loading config file: /does/not/exist
⚠️            | loading config file failed: open /does/not/exist: no such file or directory
⚠️            | attempting to load remote configuration
⚠️            | NOTE: remote configurations are an experimental feature and subject to change.
⚠️            | loading remote configuration failed: unable to fetch remote include from git: failed to clone /does/not/exist: Get "https://does/not/info/refs?service=git-upload-pack": dial tcp: lookup does: no such host
Error: failed to load image configuration: open /does/not/exist: no such file or directory
2024/01/05 14:38:51 error during command execution: failed to load image configuration: open /does/not/exist: no such file or directory