Improved handling of dlopen/LoadLibrary errors.

These are now proper errors instead of warnings, and the error message
on Windows is more informative as well, making it easier to detect and
diagnose such errors.

Backport from upstream:

https://github.com/pure-data/pure-data/commit/fee700b3
https://github.com/pure-data/pure-data/commit/af3894da

pd/src/s_loader.c

@@ -250,7 +250,14 @@ gotone:
         ntdll = LoadLibrary(filename);
         if (!ntdll)
         {
-            verbose(1, "%s: couldn't load", filename);
+            wchar_t wbuf[MAXPDSTRING];
+            char buf[MAXPDSTRING];
+            DWORD count, err = GetLastError();
+            count = FormatMessageW(FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
+                0, err, MAKELANGID (LANG_NEUTRAL, SUBLANG_DEFAULT), wbuf, MAXPDSTRING, NULL);
+            if (!count || !WideCharToMultiByte(CP_UTF8, 0, wbuf, count+1, buf, MAXPDSTRING, 0, 0))
+                *buf = '\0';
+            error("%s: %s (%d)", filename, buf, err);
             class_set_extern_dir(&s_);
             return (0);
         }
@@ -263,7 +270,7 @@ gotone:
     dlobj = dlopen(filename, RTLD_NOW | RTLD_GLOBAL);
     if (!dlobj)
     {
-        verbose(1, "%s: %s", filename, dlerror());
+        error("%s: %s", filename, dlerror());
         class_set_extern_dir(&s_);
         return (0);
     }
@@ -277,7 +284,7 @@ gotone:
 
     if (!makeout)
     {
-        verbose(1, "load_object: Symbol \"%s\" not found", symname);
+        error("load_object: Symbol \"%s\" not found", symname);
         class_set_extern_dir(&s_);
         return 0;
     }