vuepress dev <projectDir> goes ahead even if the directory doesn't exist

[open-source-explorer]

• I confirm that this is an issue rather than a question.

Bug report

Steps to reproduce

What is expected?

vuepress dev <directory> should show an appropriate warning if the directory doesn't exist.

What is actually happening?

The default template is being served.

System:
OS: Linux 4.18 Ubuntu 18.10 (Cosmic Cuttlefish)
CPU: (4) x64 Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz
Shell: 4.4.19 - /bin/bash
Binaries:
Node: 8.11.4 - /usr/bin/node
Yarn: 1.13.0 - /usr/local/bin/yarn
npm: 6.9.0 - /usr/local/bin/npm
Browsers:
Chrome: 73.0.3683.75
Firefox: 66.0.2

[open-source-explorer]

@ulivz Your thoughts?

[ulivz]

Maybe we should emit warning instead of an error since a VuePress application can be all composed with dynamic additionalPages.

[ulivz]

Updated at 6da9a5f.