report file + line number when mismatched 'nocov' comments found

[kevinushey]

E.g. from Travis, I saw:

$ Rscript -e 'covr::codecov()'
Error in parse_exclusions(x$file_lines, exclude_pattern, exclude_start,  : 
  2 starts but only 1 ends!
Calls: <Anonymous> ... package_coverage -> exclude -> lapply -> FUN -> parse_exclusions
Execution halted

Would be nice to have the file name + source line.