Add a more informative error if deps.jl is not found

[odow]

Closes #589

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.77%. Comparing base (b19d383) to head (5c4153e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #600   +/-   ##
=======================================
  Coverage   92.77%   92.77%           
=======================================
  Files           6        6           
  Lines        2658     2658           
=======================================
  Hits         2466     2466           
  Misses        192      192           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.