These errors are hard to debug because if you have a long document then add a citation to a bibtex entry which has a non-unicode character, it's pretty hard to know which citation is the problem and where to look.
My solution was to do the following:
pdflatex or whatever, at the command line. This gives you the error in "console" format, like this:
Package inputenc Error: Unicode char ́ (U+301) not set up for use with LaTeX
I could then copy/paste the offending character (in my case, a weird accent on a bit of Spanish text) and paste it into the following command:
This allowed me to find the offending item in one of my .bib files and I could make the fix and get on with my life.