Do not `fatalError` when syncKind is different than `.incremental`

[ahoppen]

Instead of crashing here, we should be reporting an error. I don’t know of any cases hitting this but we generally shouldn’t crash in SourceKit-LSP.

sourcekit-lsp/Sources/SourceKitLSP/SourceKitLSPServer.swift

Line 522 in d18aba7

fatalError("non-incremental update not implemented")