refactor: replace string-matching error checks with custom error classes

src/client.ts

@@ -12,6 +12,7 @@ import type {
 } from "vscode-languageserver-protocol";
 import * as path from "node:path";
 import type { ServerConfig } from "./types.ts";
+import { ServerNotFoundError } from "./types.ts";
 import { findRoot, pathToUri, uriToPath } from "./root.ts";
 
 // Is On PATH - Returns true if `cmd` resolves to an executable via the
@@ -73,10 +74,7 @@ export class LspClient {
     // letting spawn ENOENT surface as a generic error. It's the user's
     // responsibility to have the server installed & on PATH.
     if (!isOnPath(this.server.command)) {
-      throw new Error(
-        `LSP server binary "${this.server.command}" not found on PATH. ` +
-          `Install it and ensure it's on your PATH (required by server "${this.server.id}").`,
-      );
+      throw new ServerNotFoundError(this.server.command);
     }
     this.proc = spawn(this.server.command, this.server.args, {
       stdio: ["pipe", "pipe", "pipe"],