krun: return dlerror string when library is not found

krun returns incomplete error without this patch which is difficult to debug
```console
Error: OCI runtime error: /home/crun/krun: failed to open library `libkrun.so.1` and `libkrun-sev.so.1` for krun_config
```

after patch
```console
Error: /home/ar/work/crun/krun: failed to open library `libkrun.so.1` and `libkrun-sev.so.1` for krun_config: libkrun-sev.so.1: cannot open shared object file: No such file or directory: OCI runtime attempted to invoke a command that was not found
```

Signed-off-by: flouthoc <flouthoc.git@gmail.com>

Author: flouthoc

src/libcrun/handlers/krun.c

@@ -426,7 +426,7 @@ libkrun_load (void **cookie, libcrun_error_t *err)
   if (kconf->handle == NULL && kconf->handle_sev == NULL)
     {
       free (kconf);
-      return crun_make_error (err, 0, "failed to open `%s` and `%s` for krun_config", libkrun_so, libkrun_sev_so);
+      return crun_make_error (err, 0, "failed to open `%s` and `%s` for krun_config: %s", libkrun_so, libkrun_sev_so, dlerror());
     }
 
   kconf->sev = false;