SONARPLUGINS-3234: fail if report path doesn't exist

fix was actually to add the missing "throw" to the already present
exception creation.
general code improvements based on SonarQube (nemo) results

* Constants should be declared "final static" rather than merely "final"

* Unused formal parameter
* Java collection interfaces such as "List" should be hold instead of
specific implementations classes such as "LinkedList"
* Performance - Method concatenates strings using + in a loop