xctool crashes when setting `-reporter` without a next arg

```
2013-10-09 14:45:13.058 xctool[96700:2707] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSArrayM insertObject:atIndex:]: object cannot be nil'
*** First throw call stack:
(
	0   CoreFoundation                      0x00007fff952ee41c __exceptionPreprocess + 172
	1   libobjc.A.dylib                     0x00007fff91dd9e75 objc_exception_throw + 43
	2   CoreFoundation                      0x00007fff951af8b7 -[__NSArrayM insertObject:atIndex:] + 951
	3   xctool                              0x0000000103e9e5de -[Action consumeArguments:errorMessage:] + 719
	4   xctool                              0x0000000103e9f61f -[Options consumeArguments:errorMessage:] + 434
	5   xctool                              0x0000000103e989c8 -[XCTool run] + 491
	6   xctool                              0x0000000103e97f78 main + 507
	7   xctool                              0x0000000103e961f4 start + 52
)
libc++abi.dylib: terminating with uncaught exception of type NSException
```

This is pretty dirty. We should catch this cleanly