package.lua seems to strike a runtime exception when "requiring" a file with syntax errors. It happens because of omitted string.format() in error handling section. This issue results in exception with the traceback pointing to package.lua instead of faulty imported package file.
Problem was introduced in 31dff6e.

Files:

libPackage.lua:

if then
end

program.lua:

local lib = require("libPackage")

Production behavior:
program

Expected behavior (string.format() added to the line 68 of package.lua):
program