Fail clearly when tool has no download URL for the current platform

Tool.download() now throws IOException with a clear message when the
download URL is null (e.g. micromamba on Windows ARM64) instead of
propagating a NullPointerException from deep inside FilePaths.fileType.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ctrueden

src/main/java/org/apposed/appose/tool/Tool.java

@@ -240,6 +240,10 @@ protected void execDirect(String... args) throws IOException, InterruptedExcepti
 	}
 
 	protected File download() throws IOException, InterruptedException {
+		if (url == null) {
+			throw new IOException(name + " is not available for this platform (" +
+				Platforms.PLATFORM + "). Please install it manually.");
+		}
 		try {
 			return Downloads.download(name, url, this::updateDownloadProgress);
 		}