Log an error message when RM_CreateCommand fails parsing its flags

[gavrie]

While writing a Redis module, we called RM_CreateCommand with a typo in the strflags argument ("write " instead of "write"). This caused loading of the module to fail with a generic error message: Can't load module from <module file>: server aborting.

It took a while to understand what the actual problem was. It would have been great to see a clear error message that explained what the problem was.

This PR adds such an error message.

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