Better error reporting when JWT_SECRET is missing

[FranklinChen]

I forgot to follow the full instructions and did not set JWT_SECRET and got an infinite loop when running Yesod and was mystified for a long time before I figured out it was because I did not set JWT_SECRET. Could this app check for the missing environment variable and report that?

$ stack exec -- yesod devel
...
Starting devel application
devel.hs: Could not convert to expected type: expected Text, encountered Null
CallStack (from HasCallStack):
  error, called at src/Data/Yaml/Config.hs:201:20 in yaml-0.11.0.0-GAWf3vpOhGMK7o9d6zMyJu:Data.Yaml.Config
Unexpected: child process exited with ExitFailure 1
Trying again
Starting devel application
devel.hs: Could not convert to expected type: expected Text, encountered Null
CallStack (from HasCallStack):
  error, called at src/Data/Yaml/Config.hs:201:20 in yaml-0.11.0.0-GAWf3vpOhGMK7o9d6zMyJu:Data.Yaml.Config
Unexpected: child process exited with ExitFailure 1
Trying again
Starting devel application
devel.hs: Could not convert to expected type: expected Text, encountered Null
CallStack (from HasCallStack):
  error, called at src/Data/Yaml/Config.hs:201:20 in yaml-0.11.0.0-GAWf3vpOhGMK7o9d6zMyJu:Data.Yaml.Config
Unexpected: child process exited with ExitFailure 1
Trying again

[tzemanovic]

Thanks for the feedback! Will add that in