Add hint to error message

[karlkovaciny]

I was playing with the tutorial at https://smarie.github.io/python-pytest-steps/#c-optional-steps-and-dependencies and got this error.

I had indented badly and my code looked like this: with optional_step('step_b') as step_b: assert True yield step_b # should have been unindented, caused this error.

I just wanted to help any other newbie get past this in case it happened to them.

[codecov-io]

Codecov Report

Merging #35 into master will not change coverage. The diff coverage is 0%.

@@           Coverage Diff           @@
##           master      #35   +/-   ##
=======================================
  Coverage   83.52%   83.52%           
=======================================
  Files          27       27           
  Lines        1153     1153           
=======================================
  Hits          963      963           
  Misses        190      190

Impacted Files	Coverage Δ
pytest_steps/steps_generator.py	79.55% <0%> (ø)	:arrow_up:

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