Allow NotADirectory errors in config lookup

[tmfink]

Users may have files where directories are expected, such as $HOME/.config being a file when trying to lookup $HOME/.config/rustfmt/.rustfmt.toml. We don't want to treat such situations as errors.

$ rustfmt src/main.rs
Failed to get metadata for config file "/tmp/fake-home/.config/rustfmt/.rustfmt.toml": Not a directory (os error 20)

To reproduce the situation on a Unix-like system:

# make fake home dir
mkdir /tmp/fake-home
touch /tmp/fake-home/.config

# make test rust project
cd /tmp/
cargo new hello

# try to format
## use the path to a real rustfmt executable, not rustup
HOME=/tmp/fake-home /path/to/rustfmt /tmp/hello/src/main.rs

[fee1-dead]

Do you think we could have a test at https://github.com/rust-lang/rustfmt/blob/master/tests/rustfmt/main.rs, or some other place?