gpu-dawn: error if curl is not installed, less verbose errors

Some Linux distro's (e.g. Ubuntu) ship with wget but not curl by default. It's possible
to run into this if you don't use it a lot, e.g. in WSL under Windows - so produce an error
if `curl` is not installed.

Additionally, if the binary download fails, don't throw an entire stack trace to stdout.

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

gpu-dawn/build.zig

@@ -299,7 +299,8 @@ pub fn ensureBinaryDownloaded(
         // A download failed, or extraction failed, so wipe out the directory to ensure we correctly
         // try again next time.
         std.fs.deleteTreeAbsolute(base_cache_dir) catch {};
-        @panic(@errorName(err));
+        std.log.err("mach/gpu-dawn: prebuilt binary download failed: {s}", .{@errorName(err)});
+        std.process.exit(1);
     };
 }
 
@@ -312,6 +313,8 @@ fn downloadBinary(
     is_windows: bool,
     version: []const u8,
 ) !void {
+    try ensureCanDownloadFiles(allocator);
+
     const download_dir = try std.fs.path.join(allocator, &.{ target_cache_dir, "download" });
     try std.fs.cwd().makePath(download_dir);
 
@@ -445,6 +448,23 @@ fn downloadFile(allocator: std.mem.Allocator, target_file: []const u8, url: []co
     _ = try child.spawnAndWait();
 }
 
+fn ensureCanDownloadFiles(allocator: std.mem.Allocator) !void {
+    const argv = &[_][]const u8{ "curl", "--version" };
+    const result = try std.ChildProcess.exec(.{
+        .allocator = allocator,
+        .argv = argv,
+        .cwd = thisDir(),
+    });
+    defer {
+        allocator.free(result.stderr);
+        allocator.free(result.stdout);
+    }
+    if (result.term.Exited != 0) {
+        std.log.err("mach: error: 'curl --version' failed. Is curl not installed?", .{});
+        std.process.exit(1);
+    }
+}
+
 fn isLinuxDesktopLike(target: std.Target) bool {
     const tag = target.os.tag;
     return !tag.isDarwin() and tag != .windows and tag != .fuchsia and tag != .emscripten and !target.isAndroid();