gtk: instruct users to install blueprint-compiler (#5970)

There's been *far* too many people who aren't aware of the new
dependency, and that is partly our fault: a "FileNotFound" error is
quite obtuse, unless you religiously follow every PR and every commit
made to the repository. Instead of shepherding everyone who runs into
this manually, we should offer better signposting.

src/apprt/gtk/blueprint_compiler.zig

@@ -47,11 +47,23 @@ pub fn main() !void {
         alloc,
     );
 
-    const term = try compiler.spawnAndWait();
+    const term = compiler.spawnAndWait() catch |err| switch (err) {
+        error.FileNotFound => {
+            std.log.err(
+                \\`blueprint-compiler` not found.
+                \\
+                \\Ghostty requires `blueprint-compiler` as a build-time dependency starting from version 1.2.
+                \\Please install it, ensure that it is available on your PATH, and then retry building Ghostty.
+            , .{});
+            std.posix.exit(1);
+        },
+        else => return err,
+    };
+
     switch (term) {
         .Exited => |rc| {
-            if (rc != 0) std.posix.exit(1);
+            if (rc != 0) std.process.exit(1);
         },
-        else => std.posix.exit(1),
+        else => std.process.exit(1),
     }
 }