Improve error message "empty identifier"

[varungandhi-src]

With the attached index, I get an error

scip-ruby gem TODO TODO P#a=().: empty identifier
scip-ruby gem TODO TODO P#a=().
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scip-ruby gem TODO TODO P#w=().: empty identifier
scip-ruby gem TODO TODO P#w=().
_____________________________^

This error is not very helpful; I skimmed the code and it seems like this comes up with empty descriptor names? I put asserts in the scip-ruby code and it doesn't seem to be creating descriptors with empty names, so not sure what's up here.