Ensure source locs are always available in environment
Store Rascal stack traces in list rather than strings

* Rascal stack traces used to be passed around as strings, which
  would be difficult to change or analyse directly. This patch
  introduces a StackTrace class that stores the traces

* Stack traces are now printed so that the links should be clickable,
  and the traces should look a bit more like the Java stack traces.

  The printing style is controlled in

    src/org/rascalmpl/interpreter/StackTraceEntry.java

* Document Throw

* Exception cleanup in JDBC and ShellExec