Refactor DependencyResolver to collect and return loading errors.

This should never be triggered in production, where we always run a loading
phase first and only analyze targets that load successfully. I.e., this is
just plumbing which will be hooked up in a subsequent change.

--
MOS_MIGRATED_REVID=113258593

src/test/java/com/google/devtools/build/lib/analysis/DependencyResolverTest.java

diff --git a/src/test/java/com/google/devtools/build/lib/analysis/DependencyResolverTest.java b/src/test/java/com/google/devtools/build/lib/analysis/DependencyResolverTest.java
index 19ad653..e75ca93 100644
--- a/src/test/java/com/google/devtools/build/lib/analysis/DependencyResolverTest.java
+++ b/src/test/java/com/google/devtools/build/lib/analysis/DependencyResolverTest.java
@@ -64,6 +64,11 @@
         throw new IllegalStateException();
       }
 
+      @Override
+      protected void missingEdgeHook(Target from, Label to, NoSuchThingException e) {
+        throw new IllegalStateException(e);
+      }
+
       @Nullable
       @Override
       protected Target getTarget(Label label) throws NoSuchThingException {