Better exception handling for wagon's serve command when using -f option

[cappadona]

To catch those situations when the daemonized process is no longer running but the server.pid file is still hanging around...

# Error description:
No such process

# Backtrace:

	/usr/local/src/locomotivecms_wagon/lib/locomotive/wagon/commands/serve_command.rb:55:in `kill'
	/usr/local/src/locomotivecms_wagon/lib/locomotive/wagon/commands/serve_command.rb:55:in `stop'
	/usr/local/src/locomotivecms_wagon/lib/locomotive/wagon/commands/serve_command.rb:22:in `start'
	/usr/local/src/locomotivecms_wagon/lib/locomotive/wagon/commands/serve_command.rb:13:in `start'
	/usr/local/src/locomotivecms_wagon/lib/locomotive/wagon.rb:39:in `serve'
	/usr/local/src/locomotivecms_wagon/lib/locomotive/wagon/cli.rb:286:in `serve'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/thor-0.19.4/lib/thor/command.rb:27:in `run'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/thor-0.19.4/lib/thor/invocation.rb:126:in `invoke_command'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/thor-0.19.4/lib/thor.rb:369:in `dispatch'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/thor-0.19.4/lib/thor/base.rb:444:in `start'
	/usr/local/src/locomotivecms_wagon/bin/wagon:12:in `<top (required)>'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/bin/wagon:23:in `load'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/bin/wagon:23:in `<top (required)>'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.14.6/lib/bundler/cli/exec.rb:74:in `load'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.14.6/lib/bundler/cli/exec.rb:74:in `kernel_load'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.14.6/lib/bundler/cli/exec.rb:27:in `run'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.14.6/lib/bundler/cli.rb:335:in `exec'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.14.6/lib/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.14.6/lib/bundler/vendor/thor/lib/thor/invocation.rb:126:in `invoke_command'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.14.6/lib/bundler/vendor/thor/lib/thor.rb:359:in `dispatch'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.14.6/lib/bundler/cli.rb:20:in `dispatch'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.14.6/lib/bundler/vendor/thor/lib/thor/base.rb:440:in `start'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.14.6/lib/bundler/cli.rb:11:in `start'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.14.6/exe/bundle:32:in `block in <top (required)>'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.14.6/lib/bundler/friendly_errors.rb:121:in `with_friendly_errors'
	~/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.14.6/exe/bundle:24:in `<top (required)>'
	~/.rbenv/versions/2.3.1/bin/bundle:23:in `load'
	~/.rbenv/versions/2.3.1/bin/bundle:23:in `<main>'