Fixes a bug that applying patches to apxs2 always fails.

Hi.

I found that `Apxs2PatchTask` always failed and fixed the issue.
The error message I caught is here:

```
% ./phpbrew install http://museum.php.net/php5/php-5.4.19.tar.bz2 +default +phar +pcre +zlib +dbs +openssl +pcre +mb +apxs2=/usr/bin/apxs
===> phpbrew will now build 5.4.19
===> Loading and resolving variants...
Checking distribution checksum...
Checksum matched:
===> Distribution file was successfully extracted, skipping...
===> Applying patch - apxs2 module version name ...
apxs2 patch failed. 
```

I manually tried `perl -i.bak -pe 's#libphp\$\(PHP_MAJOR_VERSION\)\.#libphp\$\(PHP_VERSION\)\.#gx' ~/.phpbrew/build/php-5.4.19/configure`(this command was executed in Apxs2Task internally) in CLI, but the return value was 0.
I think, the problem is that `Utils::system()` returns 0 when a command succeeds, so comparing the return value with false by `!==` is always true.

Sincerely.
Passing a previous exception to `RuntimeException` makes error messages better.