Handle queryPathInfo() failure from substituters when fallback is enabled

Fixes #1990.

src/libstore/build.cc

@@ -3682,6 +3682,13 @@ void SubstitutionGoal::tryNext()
     } catch (InvalidPath &) {
         tryNext();
         return;
+    } catch (Error & e) {
+        if (settings.tryFallback) {
+            printError(e.what());
+            tryNext();
+            return;
+        }
+        throw;
     }
 
     /* Update the total expected download size. */

src/libstore/local-store.cc

@@ -880,6 +880,11 @@ void LocalStore::querySubstitutablePathInfos(const PathSet & paths,
                     narInfo ? narInfo->fileSize : 0,
                     info->narSize};
             } catch (InvalidPath) {
+            } catch (Error & e) {
+                if (settings.tryFallback)
+                    printError(e.what());
+                else
+                    throw;
             }
         }
     }