Pronk fails cryptically upon hitting a server's client request limit

Pronk does not deal with server limits:

``` shell
[vlm@nala:~/work/github/third-party/pronk (master)]> pronk -n 101  http://github.com/
pronk: ParseError {errorContexts = ["HTTP status line","demandInput"], errorMessage = "not enough bytes"}
pronk: thread blocked indefinitely in an MVar operation
[vlm@nala:~/work/github/third-party/pronk (master)]> 
```
