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_interfaces_registry_hub.h

@@ -22,6 +22,7 @@ namespace Catch {
     struct IReporterRegistry;
     struct IReporterFactory;
     struct ITagAliasRegistry;
+    class StartupExceptionRegistry;
 
     using IReporterFactoryPtr = std::shared_ptr<IReporterFactory>;
 
@@ -33,6 +34,9 @@ namespace Catch {
         virtual ITagAliasRegistry const& getTagAliasRegistry() const = 0;
 
         virtual IExceptionTranslatorRegistry& getExceptionTranslatorRegistry() = 0;
+
+
+        virtual StartupExceptionRegistry const& getStartupExceptionRegistry() const = 0;
     };
 
     struct IMutableRegistryHub {
@@ -42,6 +46,7 @@ namespace Catch {
         virtual void registerTest( TestCase const& testInfo ) = 0;
         virtual void registerTranslator( const IExceptionTranslator* translator ) = 0;
         virtual void registerTagAlias( std::string const& alias, std::string const& tag, SourceLineInfo const& lineInfo ) = 0;
+        virtual void registerStartupException( std::exception_ptr const& exception ) = 0;
     };
 
     IRegistryHub& getRegistryHub();