CLI - Exit when config is malformed

[bfops]

Description of Changes

Fixes #1668.
Fixes #1581.

API and ABI breaking changes

This will no longer overwrite malformed config files, but does not otherwise change our API.

Expected complexity level and risk

2

Testing

It would be reasonable to ask me to turn these into smoketests..

$ rm -f ~/.spacetime/config.toml

$ cargo run -- call foo bar
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.21s
     Running `target/debug/spacetime call foo bar`
Error: The dns resolution of `foo` failed.

$ cat ~/.spacetime/config.toml
default_server = "testnet"

[[server_configs]]
nickname = "local"
host = "127.0.0.1:3000"
protocol = "http"

[[server_configs]]
nickname = "testnet"
host = "testnet.spacetimedb.com"
protocol = "https"

$ sed -i 's/=/>/' ~/.spacetime/config.toml

$ cargo run -- call foo bar
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.19s
     Running `target/debug/spacetime call foo bar`
config file "/home/lead/.spacetime/config.toml" is invalid: Error {
    inner: Error {
        inner: TomlError {
            message: "expected `.`, `=`",
            original: Some(
                "default_server > \"testnet\"\n\n[[server_configs]]\nnickname > \"local\"\nhost > \"127.0.0.1:3000\"\nprotocol > \"http\"\n\n[[server_configs]]\nnickname > \"testnet\"\nhost > \"testnet.spacetimedb.com\"\nprotocol > \"https\"\n",
            ),
            keys: [],
            span: Some(
                15..16,
            ),
        },
    },
}
Error: TOML parse error at line 1, column 16
  |
1 | default_server > "testnet"
  |                ^
expected `.`, `=`

$ cat ~/.spacetime/config.toml
default_server > "testnet"

[[server_configs]]
nickname > "local"
host > "127.0.0.1:3000"
protocol > "http"

[[server_configs]]
nickname > "testnet"
host > "testnet.spacetimedb.com"
protocol > "https"