Some fix config.js

[lordofdegen]

Hey, nice to meet you! Hope you are doing well. I’ve tried fixing a few issues, and I hope my solution works for you.

##What was before

#1 Error handling when reading mnemonic file
The code reads the mnemonic file without checking if it exists or handling potential errors. If the file is missing or empty, the program crashes.

#2 Incorrect handling of mnemonics
The code tries to extract a substring from the first mnemonic without checking its length. If the string is shorter than 15 characters, it throws an error.

##What became of the fixes

#1 Now there's a file existence check and proper error handling.

#2 Now the string length is checked before calling substring.