ignore place_signs error

[mattn]

For efm-langserver, some linter command return 0 for line number. This is wrong value for Language Server Protocol. But this is useful to know that there are errors in the whole lines. So I suggest to ignore error of sign_add.

[prabirshrestha]

Shouldn't efm lang server correct it or handle it? Probably change <=0 to 1 and log on the server. If we make this fix here, it probably will only work for vim-lsp and other lsp clients.

[mattn]

I thought 0 should be 0.

[stale[bot]]

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