check for broken pipe when streaming the output.

For instance, if we give command:
````
$rust | xargs -v
````

we get,

````
xargs: illegal option -- v

usage: xargs [-0opt] [-E eofstr] [-I replstr [-R replacements]] [-J replstr]

             [-L number] [-n number [-x]] [-P maxprocs] [-s size]

             [utility [argument ...]]

error: internal compiler error: unexpected failure

This message reflects a bug in the Rust compiler.

We would appreciate a bug report: http://static.rust-lang.org/doc/master/complement-bugreport.html

note: the compiler hit an unexpected failure path. this is a bug

task '<main>' failed at 'Unhandled condition: io_error: io::IoError{kind: BrokenPipe, desc: "broken pipe", detail: None}', /Users/abhay/devel/rust/rust/src/libstd/condition.rs:139
````

The `BrokenPipe` (or `EPIPE`) occurs when the receiving-end process returns non-zero exit-status.

I tested raising the condition with following `C` code:

````
//C File: test_line.c
#include <stdio.h>

int main() {
	char *line = NULL;
	size_t linecap = 0;
	ssize_t linelen;
	
	while ((linelen = getline(&line, &linecap, stdin)) > 0)
		fwrite(line, linelen, 1, stdout);	
	
	return 1;
}

````
The above code returns 1 after reading everything from the piped process.
The commands i executed:

````
$clang test_line.c -o test_line
$rustc | ./test_line
````

The last command gave the `unexpected failure` error.

This patch checks if raised condition is not BrokenPipe, and otherwise, it raised the condition.