[#608] Don't use exit() on duplicate test descriptions

Instead of `exit(1)`, it now throws `std::runtime_error` with the details
of the failure. This exception is handled in `run()` at a higher level where
the log is printed to cerr and the test gracefully exits.

include/internal/catch_test_case_registry_impl.hpp

@@ -61,12 +61,14 @@ namespace Catch {
             ++it ) {
             std::pair<std::set<TestCase>::const_iterator, bool> prev = seenFunctions.insert( *it );
             if( !prev.second ) {
-                Catch::cerr()
+                std::ostringstream ss;
-                << Colour( Colour::Red )
+
+                ss  << Colour( Colour::Red )
                     << "error: TEST_CASE( \"" << it->name << "\" ) already defined.\n"
                     << "\tFirst seen at " << prev.first->getTestCaseInfo().lineInfo << "\n"
                     << "\tRedefined at " << it->getTestCaseInfo().lineInfo << std::endl;
-                exit(1);
+
+                throw std::runtime_error(ss.str());
             }
         }
     }