decoding a keystore file can fail unexpectedly

[michaelsbradleyjr]

This was discovered in the example client when rapidly running valid /sendtransaction ... commands without waiting for the previous one/s to finish.

Sometimes in the TUI there would be an error displayed re: failed decoding of a key file. It can take several runs to reproduce as the crash reported in #290 usually happens first.

The cause of the failure is currently unknown. It may have to do with a failure during json = parseFile(path) in loadAccount of private/accounts/generator/generator.nim. If so, then it might be related to the parseJson problem reported in #290.