threadpool: catch exceptions in dtor, to avoid terminate

If an exception is thrown, it is ignored. While this may hide
a bug, this should only be system exceptions in boost, which
is pretty unlikely. Morever, wait should be called manually
before the dtor anyway. Add an error message if the dtor has
to wait in case some such cases creep in so they get fixed.

Coverity 182538

src/common/threadpool.h

@@ -34,6 +34,7 @@
 #include <functional>
 #include <utility>
 #include <vector>
+#include <stdexcept>
 
 namespace tools
 {
@@ -57,7 +58,7 @@ public:
     void dec();
     void wait();  //! Wait for a set of tasks to finish.
     waiter() : num(0){}
-    ~waiter() { wait(); }
+    ~waiter();
   };
 
   // Submit a task to the pool. The waiter pointer may be