Give more details when plugin discovery fails for a legit reason.

Skip files the whose mime type is not application/x-sharedlib,
which would be the ones allowed to fail loading. All errors on
failed dlopen() attempts now cause the code to throw.

include/backends/backend_loader.hpp

@@ -21,6 +21,7 @@
 #include <string>
 #include <memory>
 #include <boost/utility/string_ref.hpp>
+#include <exception>
 
 namespace YAML {
 	class Node;
@@ -31,6 +32,11 @@ namespace dindb {
 
 	using BackendPtr = std::unique_ptr<dindb::Backend, void(*)(dindb::Backend*)>;
 
+	class SOLoadException : public std::runtime_error {
+	public:
+		explicit SOLoadException (std::string&& parMessage);
+	};
+
 	class BackendPlugin {
 	public:
 		BackendPlugin ( void );