Server should validate esbonio.server.buildCommand

[alcarney]

Expected behavior

e.g. When given the full path to the sphinx-build executable, the server should either reject it, or strip the path and perhaps emit a warning. It would also be good to check how the server handles invalid options

see: #778 (comment)

Actual behavior

The server accepts the invalid option and throws cryptic errors

Log output

No response

(Optional) Settings from conf.py

No response