allow cat of regression diff to fail, print a warning instead

this will allow the regression test results to all be reported even if there is such a huge difference between the dynamic and baseline that the system utilities cannot print it

see: #2316 (comment)

.github/workflows/CI.yml

@@ -272,7 +272,7 @@ jobs:
               export FAILED=Yes
             fi
             echo "" # blank line so next block is interpreted as markdown
-            cat "$regr_test-core.log"
+            cat "$regr_test-core.log" || (echo "Dumping the whole log failed, please download it from GitHub actions. Here are the first 100 lines:" && head -n100 "$regr_test-core.log")
             echo "</details>"
             echo "<details>"
             if python-jl scripts/checkModels.py \
@@ -289,7 +289,7 @@ jobs:
               export FAILED=Yes
             fi
             echo "" # blank line so next block is interpreted as markdown
-            cat "$regr_test-edge.log"
+            cat "$regr_test-edge.log" || (echo "Dumping the whole log failed, please download it from GitHub actions. Here are the first 100 lines:" && head -n100 "$regr_test-core.log")
             echo "</details>"
 
             # Check for Regression between Reference and Dynamic (skip superminimal)