Fixes NPE when coverage.xml has multiple <source> element

Coverage report can have multiple source folders

```xml
	<sources>
		<source>src/main/java</source>
		<source>src/main/groovy</source>
	</sources>
```

Aside of that Gradle's Cobertura plugin can generate an additional wrong source folder.

So this PR 
* find the first matching source in any of the source directories
* makes this plugin more lenient of the actual source file cannot be found

Also I'd like to thank Adrien Lecharpentier (@alecharp) who helped me to coin this bug