Cabal silently ignores parsing errors in config file.

I was installing `network` library on Windows and discovered the following.
I have this line in my global config file:

```
extra-prog-path: C:\Users\alex\AppData\Roaming\cabal\bin; C:\Program Files\Haskell Platform\8.0.1\msys\usr\bin
```

Note the space character after semicolon. Cabal seems to pick up only the first part of the path, and installation fails with

```
cabal.exe: The package has a './configure' script. If you are on Windows, This
requires a Unix compatibility toolchain such as MinGW+MSYS or Cygwin. If you
are not on Windows, ensure that an 'sh' command is discoverable in your path.
```

If I remove that space character everything works fine.