vim.notify cannot be suppressed and it is not always necessary to
display a visible warning to the user if the RPC process fails to start.
For instance, a user may have the same LSP configuration across systems,
some of which may not have all of the LSP server executables installed.
In that case, the user receives a notification every time a file is
opened that they cannot suppress.
Instead of using vim.notify in vim.lsp.rpc, propagate a normal error up
through the call stack and use vim.notify in vim.lsp.start() only if
the "silent" option is not set.
This also updates lsp.start_client() to return an error message as its
second return value if an error occurred, rather than calling vim.notify
directly. Callers of lsp.start_client() will need to update call sites
appropriately if they wish to report errors to the user (or even better,
switch to vim.lsp.start).
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