Adding additional logging that would have been useful when debugging issue #152.

--
Change-Id: I9968c3da2210b630502b47b7ed96acd71c7d7349
MOS_MIGRATED_REVID=92749744

src/main/java/com/google/devtools/build/lib/actions/Artifact.java

diff --git a/src/main/java/com/google/devtools/build/lib/actions/Artifact.java b/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
index 4eb11ca..c39c228 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
@@ -139,10 +139,12 @@
   @VisibleForTesting
   public Artifact(Path path, Root root, PathFragment execPath, ArtifactOwner owner) {
     if (root == null || !path.startsWith(root.getPath())) {
-      throw new IllegalArgumentException(root + ": illegal root for " + path);
+      throw new IllegalArgumentException(root + ": illegal root for " + path
+          + " (execPath: " + execPath + ")");
     }
     if (execPath == null || execPath.isAbsolute() || !path.asFragment().endsWith(execPath)) {
-      throw new IllegalArgumentException(execPath + ": illegal execPath for " + path);
+      throw new IllegalArgumentException(execPath + ": illegal execPath for " + path
+          + " (root: " + root + ")");
     }
     this.path = path;
     this.root = root;