Collect startup exceptions instead of throwing them

Previously, some errors in Catch configuration would cause exceptions to
be thrown before main was even entered. This leads to call to
`std::terminate`, which is not a particularly nice way of ending the
binary.

Now these exceptions are registered with a global collector and used
once Catch enters main. They can also be optionally ignored, if user
supplies his own main and opts not to check them (or ignored them
intentionally).

Closes #921

include/internal/catch_startup_exception_registry.h

@@ -0,0 +1,27 @@
+/*
+ *  Created by Martin on 04/06/2017.
+ *  Copyright 2017 Two Blue Cubes Ltd. All rights reserved.
+ *
+ *  Distributed under the Boost Software License, Version 1.0. (See accompanying
+ *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ */
+#ifndef TWOBLUECUBES_CATCH_STARTUP_EXCEPTION_REGISTRY_H_INCLUDED
+#define TWOBLUECUBES_CATCH_STARTUP_EXCEPTION_REGISTRY_H_INCLUDED
+
+
+#include <vector>
+#include <exception>
+
+namespace Catch {
+
+    class StartupExceptionRegistry {
+    public:
+        void add(std::exception_ptr const& exception);
+        std::vector<std::exception_ptr> const& getExceptions() const;
+    private:
+        std::vector<std::exception_ptr> m_exceptions;
+    };
+
+} // end namespace Catch
+
+#endif // TWOBLUECUBES_CATCH_STARTUP_EXCEPTION_REGISTRY_H_INCLUDED