Guard against format requests for files that don't yet exist on disk

[xeger]

Steps to reproduce:

1. Install vscode-syntax-tree & make sure it's in working order on existing source files
2. Use code foo.rb to open a buffer with a name, on a file that doesn't yet exist on disk
3. Type some Ruby code into the buffer to trigger a textDocument/inlayHints request

Outcome: the stree process crashes with ENOENT

Desired: the process recovers from file-not-exist (either by failing the jsonrpc request, or perhaps by grabbing fresh content from the request and putting it into the store).

Full backtrace here:

<snip>/syntax_tree-2.8.0/lib/syntax_tree/language_server.rb:26:in `binread': No such file or directory @ rb_sysopen - /tmp/bar.rb (Errno::ENOENT)
	from <snip>/syntax_tree-2.8.0/lib/syntax_tree/language_server.rb:26:in `block in run'
	from <snip>/syntax_tree-2.8.0/lib/syntax_tree/language_server.rb:68:in `run'
	from <snip>/syntax_tree-2.8.0/lib/syntax_tree/cli.rb:262:in `run'
	from <snip>/syntax_tree-2.8.0/exe/stree:9:in `<top (required)>'

[kddnewton]

Tangential but related to this is #103, because that should make it so that these requests never get fired in the first place.