add warning to mkerr.pl in case a new crypto sub-library is not represented in openssl.ec

[DDvO]

When adding a sub-library , among others the file crypto/err/openssl.ec needs to be extended as described in crypto/err/README.
It may happen that this is forgotten or the new entry gets lost. Then util/mkerr.pl will simply not add the error declarations for the new sub-library, which later will lead to compilation failures. Such an omission so far is hard to debug - for instance, I spent more than an hour trying to find out why the new error symbols were not generated any more (after an inadvertent reset of crypto/err/openssl.ec).
For this reason I propose to add a warning like the one contained in this PR.

[levitte]

I'm sorry, just the list of exclusions makes this a bad idea. I do like the initiative, though, and did try to tinker an alternative, only to find that it generates too many false positives. However, util/ck_errf.pl is much easier to adapt to do what you want... I've a counter-PR coming up.

[levitte]

Counter proposal in #6455

[levitte]

#6455 got accepted, please try it out... (closing this)

[DDvO]

I did try it out and can confirm it solved my problem.