cpus: Improve error messages on memsave, pmemsave write error

qmp_memsave() and qmp_pmemsave() report fwrite() error as

    An IO error has occurred

Improve this to

    writing memory to '<filename>' failed

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240513141703.549874-5-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

system/cpus.c

@@ -813,7 +813,8 @@ void qmp_memsave(int64_t addr, int64_t size, const char *filename,
             goto exit;
         }
         if (fwrite(buf, 1, l, f) != l) {
-            error_setg(errp, QERR_IO_ERROR);
+            error_setg(errp, "writing memory to '%s' failed",
+                       filename);
             goto exit;
         }
         addr += l;
@@ -843,7 +844,8 @@ void qmp_pmemsave(int64_t addr, int64_t size, const char *filename,
             l = size;
         cpu_physical_memory_read(addr, buf, l);
         if (fwrite(buf, 1, l, f) != l) {
-            error_setg(errp, QERR_IO_ERROR);
+            error_setg(errp, "writing memory to '%s' failed",
+                       filename);
             goto exit;
         }
         addr += l;