Better error should be shown when Namecoin Core is still syncing

[JeremyRand]

Namecoin Core gives this error when name_show is issued during syncup, so that the user doesn't unexpectedly get old data:

Namecoin is downloading blocks... (code -10)

Unfortunately NMControl isn't aware of this error (it doesn't happen in NamecoinQ), which causes getValue to return False, which isn't a useful error, particularly since a nonexistent name will also (I think) return False.

I suggest raising an exception in the namecoin and rest backends when this is encountered, so that NMControl returns the exception.

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