if decryption fails for the user because of a bad/incorrectly set DOTENV_KEY give the user better help

[motdotla]

for example tell them to run echo $DOTENV_KEY so they can view the current value. a friend had a hard time debugging this. turns out they had a global DOTENV_KEY set in their zshrc file. as soon as i got on the phone with them and told them to run echo $DOTENV_KEY to see what it looked like, their brain instantly remember they had hardcoded it in their zshrc file. adding that sort of help might help others connect the dots faster in their minds like that.

[motdotla]

this lives in dotenv. this PR will help when released: motdotla/dotenv#795

[motdotla]

that PR is ready so will update dotenvx to make use of it soon

[motdotla]

#40