User-friendly exception if root flow does not override the run() method

[awaelchli]

What does this PR do?

Part of #14677
Similar to #14759

Does your PR introduce any breaking changes? If yes, please list them.

No.

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Anyone in the community is free to review the PR once the tests have passed.
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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda

[codecov]

Codecov Report

Merging #14760 (9b50dcb) into master (35c65b0) will increase coverage by 1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #14760     +/-   ##
=========================================
+ Coverage      84%      85%     +1%     
=========================================
  Files         395      328     -67     
  Lines       28805    25777   -3028     
=========================================
- Hits        24087    21859   -2228     
+ Misses       4718     3918    -800     

[tchaton]

LGTM !

[krshrimali]

Moving the milestone to 0.6.x, as this is a helpful error. @awaelchli @tchaton - Is it okay? Feel free to move it back if 0.7 is the desired one.

[awaelchli]

If this was for Trainer, we wouldn't add it to the patch release, because it is not a bugfix. I don't know how strict we are on the app side, so redirecting this question to @Borda @tchaton.