MXS-1929: Report runtime module loading errors to users

If a module is not found, a proper error message is now given to the
user. Other system errors are also printed if errno is set. Dynamic
library errors aren't printed as they are apparently reset after a call to
`dlerror`, at least according to documentation.

server/core/config_runtime.cc

@@ -124,8 +124,8 @@ static std::pair<bool, MXS_CONFIG_PARAMETER*> load_defaults(const char* name,
     }
     else
     {
-        MXS_ERROR("Failed to load module '%s'. See previous error messages for "
-                  "more details.", name);
+        config_runtime_error("Failed to load module '%s': %s", name,
+                             errno ? mxs_strerror(errno) : "See MaxScale logs for details");
     }
 
     return {rval, params};