Stop hiding exception raised by composer

composer/lib/dependabot/composer/update_checker/version_resolver.rb

@@ -307,7 +307,10 @@ def handle_composer_errors(error)
 
             # If there *is* a lockfile we can't confidently distinguish between
             # cases where we can't install and cases where we can't update. For
-            # now, we therefore just ignore the dependency.
+            # now, we therefore just ignore the dependency and log the error.
+
+            Dependabot.logger.error(error.message)
+            error.backtrace.each { |line| Dependabot.logger.error(line) }
             nil
           elsif error.message.include?("URL required authentication") ||
                 error.message.include?("403 Forbidden")

composer/spec/dependabot/composer/update_checker_spec.rb

@@ -513,7 +513,7 @@
       end
     end
 
-    context "with an update that can't resolve" do
+    context "with an update that can't resolve due to a version conflict" do
       let(:project_name) { "version_conflict_on_update" }
       let(:dependency_name) { "longman/telegram-bot" }
       let(:dependency_version) { "2.1.5" }
@@ -528,6 +528,19 @@
 
       it { is_expected.to be_nil }
 
+      it "logs an error" do
+        allow(Dependabot.logger).to receive(:error)
+
+        is_expected.to be_nil
+        expect(Dependabot.logger).to have_received(:error).with(
+          a_string_starting_with("Your requirements could not be resolved to an installable set of packages.")
+        ).once
+
+        expect(Dependabot.logger).to have_received(:error).with(
+          a_string_starting_with("/home/dependabot/")
+        ).at_least(:once)
+      end
+
       context "and there is no lockfile" do
         let(:project_name) { "version_conflict_on_update_without_lockfile" }