Use return() instead of exit()

Previously, the function employed `exit()`, which, when invoked, results in the immediate termination of the entire program. In a library this is not a wanted response when handling errors, a better approach is to return an error code and let  the calling application handle the error.

(cherry picked from commit 477c9cd)
Signed-off-by: Ted Ralphs <ted@lehigh.edu>

Osi/src/Osi/OsiSolverInterface.cpp

@@ -1418,7 +1418,7 @@ int OsiSolverInterface::writeLpNative(const char *filename,
   if (!fp) {
     printf("### ERROR: in OsiSolverInterface::writeLpNative(): unable to open file %s\n",
       filename);
-    exit(1);
+    return (1);
   }
   int nerr = writeLpNative(fp, rowNames, columnNames,
     epsilon, numberAcross, decimals,