fix(startup): ignore broken $XDG_RUNTIME_DIR #30285

Problem:
$XDG_RUNTIME_DIR may be broken on WSL, which prevents starting (and even
building) Nvim. #30282

Solution:
- When startup fails, mention the servername in the error message.
- If an autogenerated server address fails, log an error and continue
  with an empty `v:servername`. It's only fatal if a user provides a bad
  `--listen` or `$NVIM_LISTEN_ADDRESS` address.

Before:

    $ nvim --headless --listen ./hello.sock
    nvim: Failed to --listen: "address already in use"
    $ NVIM_LISTEN_ADDRESS='./hello.sock' ./build/bin/nvim --headless
    nvim: Failed to --listen: "address already in use"

After:

    $ nvim --headless --listen ./hello.sock
    nvim: Failed to --listen: address already in use: "./hello.sock"
    $ NVIM_LISTEN_ADDRESS='./hello.sock' ./build/bin/nvim --headless
    nvim: Failed $NVIM_LISTEN_ADDRESS: address already in use: "./hello.sock"

0 files changed, 0 insertions, 0 deletions