一、注释

Try to load a module.

Will not throw an error if the module was not found, but will throw an error if the loading failed for another reason

二、原因

require加载模块时，加载错误就会报错，不区别对待下面两种情况

模块找不到
模块中的代码加载错误

load_module_if_exists()这个函数对这两种情况做了区别，下面举例说明一下：

模块找不到

$ lua -e "zz = require('zz')"
lua: (command line):1: module 'zz' not found:
    no field package.preload['zz']
    no file '/usr/local/share/lua/5.2/zz.lua'
    no file '/usr/local/share/lua/5.2/zz/init.lua'
    no file '/usr/local/lib/lua/5.2/zz.lua'
    no file '/usr/local/lib/lua/5.2/zz/init.lua'
    no file './zz.lua'
    no file '/usr/local/lib/lua/5.2/zz.so'
    no file '/usr/local/lib/lua/5.2/loadall.so'
    no file './zz.so'
stack traceback:
    [C]: in function 'require'
    (command line):1: in main chunk
    [C]: in ?

模块代码加载语法错误

lua -e "require('zz')"                                                                                                            
lua: ./zz.lua:5: attempt to perform arithmetic on field 'a' (a nil value)
stack traceback:
    ./zz.lua:5: in main chunk
    [C]: in function 'require'
    (command line):1: in main chunk
    [C]: in ?