Emit suggestions and an explanation when CBMC runs out of memory (rust-lang#2885)

Improves the error reported to the user when the system kills CBMC to
reclaim memory.

Resolves rust-lang#2715

Author: JustusAdam

kani-driver/src/call_cbmc.rs

@@ -327,8 +327,17 @@ impl VerificationResult {
             }
             Err(exit_status) => {
                 let verification_result = console::style("FAILED").red();
+                let explanation = if *exit_status == 137 {
+                    "CBMC appears to have run out of memory. You may want to rerun your proof in \
+                    an environment with additional memory or use stubbing to reduce the size of the \
+                    code the verifier reasons about.\n"
+                } else {
+                    ""
+                };
                 format!(
-                    "\nCBMC failed with status {exit_status}\nVERIFICATION:- {verification_result}\n",
+                    "\nCBMC failed with status {exit_status}\n\
+                    VERIFICATION:- {verification_result}\n\
+                    {explanation}",
                 )
             }
         }