real_dlsym: Crash out with an error if we cannot find the real dlsym

Simon McVittierequested to merge
smcv/libstrangle:diagnose-fatal-errors into master

Previously, libstrangle would crash immediately after this with a null pointer dereference (segmentation fault). There's no real way to recover in this situation, so failure is the only option, but printing an error message on stderr and aborting with a core dump seems like a more user-friendly way to fail.

This includes !22 (merged), but in fact it's how I diagnosed that !22 (merged) was necessary. They can be applied in either order with a bit of merge-conflict resolution.

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