patch 8.2.3208: dynamic library load error does not mention why it failed

Problem:    Dynamic library load error does not mention why it failed.
Solution:   Add the error message. (Martin Tournoij, closes #8621)
diff --git a/src/if_python.c b/src/if_python.c
index 7f90ede..a270982 100644
--- a/src/if_python.c
+++ b/src/if_python.c
@@ -141,10 +141,12 @@
 #  endif
 #  define close_dll dlclose
 #  define symbol_from_dll dlsym
+#  define load_dll_error dlerror
 # else
 #  define load_dll vimLoadLib
 #  define close_dll FreeLibrary
 #  define symbol_from_dll GetProcAddress
+#  define load_dll_error GetWin32Error
 # endif
 
 // This makes if_python.c compile without warnings against Python 2.5
@@ -688,7 +690,7 @@
     if (!hinstPython)
     {
 	if (verbose)
-	    semsg(_(e_loadlib), libname);
+	    semsg(_(e_loadlib), libname, load_dll_error());
 	return FAIL;
     }