[introspection] Improve error message when failing to find a native library. (#2573)

tests/introspection/ApiFieldTest.cs

@@ -232,7 +232,10 @@ namespace Introspection {
 
 				string path = FindLibrary (f.LibraryName);
 				IntPtr lib = Dlfcn.dlopen (path, 0);
-				if (Dlfcn.GetIndirect (lib, name) == IntPtr.Zero) {
+				if (lib == IntPtr.Zero) {
+					ReportError ("Could not open the library '{0}' to find the field '{1}': {2}", path, name, Dlfcn.dlerror ());
+					failed_fields.Add (name);
+				} else if (Dlfcn.GetIndirect (lib, name) == IntPtr.Zero) {
 					ReportError ("Could not find the field '{0}' in {1}", name, path);
 					failed_fields.Add (name);
 				}