Make missing TR mapping namespace throw an exception instead of printing to stderr

src/main/java/net/fabricmc/loom/util/MappingException.java

@@ -22,7 +22,7 @@
  * SOFTWARE.
  */
 
-package net.fabricmc.loom.util.srg;
+package net.fabricmc.loom.util;
 
 /**
  * An exception that occurs when processing obfuscation mappings.

src/main/java/net/fabricmc/loom/util/TinyRemapperHelper.java

@@ -155,7 +155,10 @@ public final class TinyRemapperHelper {
 			final int toId = mappings.getNamespaceId(to);
 
 			if (toId == MappingTreeView.NULL_NAMESPACE_ID) {
-				System.out.println("Warning: Trying to remap to unknown namespace: " + to);
+				throw new MappingException(
+						"Trying to remap from '%s' (id: %d) to unknown namespace '%s'. Available namespaces: [%s -> %s]"
+								.formatted(from, fromId, to, mappings.getSrcNamespace(), String.join(", ", mappings.getDstNamespaces()))
+				);
 			}
 
 			for (MappingTree.ClassMapping classDef : mappings.getClasses()) {

src/main/java/net/fabricmc/loom/util/srg/SrgMerger.java

@@ -42,6 +42,7 @@ import com.google.common.collect.ListMultimap;
 import org.jetbrains.annotations.Nullable;
 
 import net.fabricmc.loom.api.mappings.layered.MappingsNamespace;
+import net.fabricmc.loom.util.MappingException;
 import net.fabricmc.loom.util.function.CollectionUtil;
 import net.fabricmc.mappingio.FlatMappingVisitor;
 import net.fabricmc.mappingio.MappingReader;