Replace echo-like messages for syntax errors with quickfix lists or diagnostic highlights

[novusnota]

Or with a pop-up window, see:

• https://vimhelp.org/popup.txt.html
• https://vi.stackexchange.com/questions/24462/what-are-the-new-popup-windows-in-vim-8-2
• https://github.com/prabirshrestha/vim-lsp/blob/f7ccf006df1aefd327c0e2c55cc8632a2db577c1/autoload/lsp/ui/vim/output.vim

highlighting quickfix errors inline and adding signs:

• https://www.reddit.com/r/vim/comments/2vw4ts/quickfix_buffer_error_highlighting/
• https://github.com/bfrg/vim-qf-diagnostics
• https://vi.stackexchange.com/questions/25035/highlight-results-of-grep-while-in-quickfix-window
• https://github.com/jceb/vim-hier
• https://github.com/MarcWeber/vim-addon-signs
• https://github.com/dhruvasagar/vim-markify