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"],

src/root.ts

@@ -3,6 +3,7 @@ import * as path from "node:path";
 import { pathToFileURL, fileURLToPath } from "node:url";
 import { servers } from "../server.ts";
 import type { ServerConfig } from "./types.ts";
+import { UnsupportedExtensionError } from "./types.ts";
 
 // Resolve File URI To Local Path
 export function uriToPath(uri: string): string {
@@ -20,7 +21,7 @@ export function pickServer(filePath: string): ServerConfig {
   const ext = path.extname(filePath).replace(/^\./, "");
   const hit = servers.find((s) => s.match.includes(ext));
   if (!hit) {
-    throw new Error(`No LSP server registered for extension ".${ext}"`);
+    throw new UnsupportedExtensionError(`.${ext}`);
   }
   return hit;
 }

src/types.ts

@@ -1,3 +1,22 @@
+// LSP Errors - Custom error classes so callers can distinguish expected
+// conditions (unsupported file type, missing binary) from unexpected ones.
+export class UnsupportedExtensionError extends Error {
+  constructor(ext: string) {
+    super(`No LSP server registered for extension "${ext}"`);
+    this.name = "UnsupportedExtensionError";
+  }
+}
+
+export class ServerNotFoundError extends Error {
+  constructor(command: string) {
+    super(
+      `LSP server binary "${command}" not found on PATH. ` +
+        `Install it and ensure it's on your PATH.`,
+    );
+    this.name = "ServerNotFoundError";
+  }
+}
+
 export interface ServerConfig {
   // Stable identifier (useful for logs and future daemon cache keys).
   id: string;