MySQL 5.7 No Longer Returns Meaningful Error Messages
Looking through the MySQL error log, I noticed the following line: Can't find error-message file '\share\errmsg.sys'. Check error-message file location and 'lc-messages-dir' configuration directive. That file did not exist, however \share\english\errmsg.sys did exist. I found out from here that this error message is not entirely accurate, since MySQL does not actually look in the \share\ directory - it looks in the \share\*YOUR LANGUAGE*\ directory.

I looked in my my.ini and discovered that I had a line that said basedir = (that's all that the line said - there was no path after basedir). I'm not entirely sure how that configuration came to be, but I commented out that line and everything works great now!