Fail on unknown brew subcommand

Previously brew would return a 0 exit code and no output if someone
typos a command.

Before:

```console
❯ brew instlal

❯ echo $?
0
```

After:

```console
❯ brew instlal
Example usage:
  brew search TEXT|/REGEX/
  brew info [FORMULA|CASK...]
  brew install FORMULA|CASK...
  brew update
  brew upgrade [FORMULA|CASK...]
  brew uninstall FORMULA|CASK...
  brew list [FORMULA|CASK...]

Troubleshooting:
  brew config
  brew doctor
  brew install --verbose --debug FORMULA|CASK

Contributing:
  brew create URL [--no-fetch]
  brew edit [FORMULA|CASK...]

Further help:
  brew commands
  brew help [COMMAND]
  man brew
  https://docs.brew.sh

Error: Invalid usage: Unknown command: brew instlal

❯ echo $?
1
```

Author: gibfahn

Library/Homebrew/brew.rb

@@ -120,6 +120,8 @@
       ENV["HOMEBREW_#{env}"] = Object.const_get(:"HOMEBREW_#{env}").to_s
     end
     exec "brew-#{cmd}", *ARGV
+  else
+    raise UsageError, "Unknown command: brew #{cmd}"
   end
 rescue UsageError => e
   require "help"