Fix CRF decompile failing on c0.30 (#1384)

- ClassLineNumbers.readMappings throws a nicer error when the input data is empty.
- GenerateSourcesTask include linemap filename when failed to read.
- CRF decompiler wont write empty linemap data.
- Add test

src/main/java/net/fabricmc/loom/task/GenerateSourcesTask.java

@@ -631,6 +631,8 @@ public abstract class GenerateSourcesTask extends AbstractLoomTask {
 
 		try (BufferedReader reader = Files.newBufferedReader(linemapFile, StandardCharsets.UTF_8)) {
 			return ClassLineNumbers.readMappings(reader);
+		} catch (Exception e) {
+			throw new IOException("Failed to read line number map: " + linemapFile, e);
 		}
 	}