Report failed launches as exit code 127 instead of throwing

With direct argv, a missing or non-executable command makes
ProcessBuilder.start() throw, where the removed shell wrapper used to
start and exit with an error code. Callers (e.g., ProcessExecution,
SpecsGraphviz dot detection) relied on receiving a non-zero return
value instead of an exception, so mirror the shell behavior: return a
ProcessOutput with exit code 127 and the failure message on stderr.

Author: lm-sousa

SpecsUtils/src/pt/up/fe/specs/util/SpecsSystem.java

@@ -23,10 +23,12 @@
 import pt.up.fe.specs.util.utilities.JarPath;
 import pt.up.fe.specs.util.utilities.ProgressCounter;
 
+import java.io.ByteArrayInputStream;
 import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
+import java.nio.charset.StandardCharsets;
 import java.lang.invoke.MethodHandles;
 import java.lang.invoke.MethodHandles.Lookup;
 import java.lang.invoke.VarHandle;
@@ -190,10 +192,20 @@ public static <O, E> ProcessOutput<O, E> runProcess(ProcessBuilder builder,
 
         Process process;
         try {
-        process = builder.start();
+            process = builder.start();
 
         } catch (IOException e) {
-            throw new RuntimeException("Could not start process", e);
+            // The command could not be executed (e.g., not found, not executable).
+            // A shell launcher would have started and exited with an error code
+            // (127 for not found), so report failure the same way instead of throwing.
+            SpecsLogs.msgInfo("Could not start process: " + e.getMessage());
+            try (InputStream emptyOut = new ByteArrayInputStream(new byte[0]);
+                    InputStream emptyErr = new ByteArrayInputStream(
+                            ("Could not start process: " + e.getMessage()).getBytes(StandardCharsets.UTF_8))) {
+                return new ProcessOutput<>(127, outputProcessor.apply(emptyOut), errorProcessor.apply(emptyErr));
+            } catch (IOException e2) {
+                throw new RuntimeException("Could not build error output for failed launch", e2);
+            }
         }
 
         ExecutorService stdoutThread = Executors.newSingleThreadExecutor();