gtk: instruct users to install blueprint-compiler

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),
     }
 }