Problem:
LSP error responses implicitly rely on a custom `__tostring` function
(`vim.lsp.rpc.format_rpc_error`) for formatting. This causes errors that are not
created via `vim.lsp.rpc.error` to behave inconsistently with those that are.
Furthermore, we usually use `log.error` to print these errors, which uses
`vim.inspect` under the hood, so the custom `__tostring` provides little
benefit.
This increases the difficulty of refactoring the code, as it tightly couples RPC
error handling with the LSP.
Solution:
Convert every potential `__tostring` call to an explicit one. Since we don't
describe this behavior in the documentation, this should not be a breaking
change.
0eb2eb4106
