Detect and warn about broken symlinks

[tfoote]

If you have a broken symlink inside of the working directory jekyll will crash.

/var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/utils.rb:141:in initialize': No such file or directory @ rb_sysopen - /tmp/jekyll/xauth-1000-_1 (Errno::ENOENT)`

It would be helpful to detect this and give a more useful error message. As mounting the docker volume can break symlinks outside of the directory when invoked.

A traceback looks like this

docker run --rm -it -w /tmp/jekyll -v /home/tfoote/work/github/osrf/roscon2022-website:/tmp/jekyll -p 0.0.0.0:4000:4000 --network host  c42668237172 jekyll serve -w
WARNING: Published ports are discarded when using host network mode
Configuration file: /tmp/jekyll/_config.yml
            Source: /tmp/jekyll
       Destination: /tmp/jekyll/2022
 Incremental build: disabled. Enable with --incremental
      Generating... 
                    ------------------------------------------------
      Jekyll 4.2.1   Please append `--trace` to the `serve` command 
                     for any additional information or backtrace. 
                    ------------------------------------------------
Traceback (most recent call last):
	23: from /usr/local/bin/jekyll:23:in `<main>'
	22: from /usr/local/bin/jekyll:23:in `load'
	21: from /var/lib/gems/2.7.0/gems/jekyll-4.2.1/exe/jekyll:15:in `<top (required)>'
	20: from /var/lib/gems/2.7.0/gems/mercenary-0.4.0/lib/mercenary.rb:21:in `program'
	19: from /var/lib/gems/2.7.0/gems/mercenary-0.4.0/lib/mercenary/program.rb:44:in `go'
	18: from /var/lib/gems/2.7.0/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `execute'
	17: from /var/lib/gems/2.7.0/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `each'
	16: from /var/lib/gems/2.7.0/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `block in execute'
	15: from /var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/commands/serve.rb:86:in `block (2 levels) in init_with_program'
	14: from /var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/command.rb:91:in `process_with_graceful_fail'
	13: from /var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/command.rb:91:in `each'
	12: from /var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/command.rb:91:in `block in process_with_graceful_fail'
	11: from /var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/commands/build.rb:36:in `process'
	10: from /var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/commands/build.rb:65:in `build'
	 9: from /var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/command.rb:28:in `process_site'
	 8: from /var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/site.rb:78:in `process'
	 7: from /var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/site.rb:181:in `read'
	 6: from /var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/reader.rb:16:in `read'
	 5: from /var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/reader.rb:48:in `read_directories'
	 4: from /var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/reader.rb:48:in `each'
	 3: from /var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/reader.rb:52:in `block in read_directories'
	 2: from /var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/utils.rb:141:in `has_yaml_header?'
	 1: from /var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/utils.rb:141:in `open'
/var/lib/gems/2.7.0/gems/jekyll-4.2.1/lib/jekyll/utils.rb:141:in `initialize': No such file or directory @ rb_sysopen - /tmp/jekyll/xauth-1000-_1 (Errno::ENOENT)