ShellManager: catch exception thrown by env var functors

src/shell_manager.cc

@@ -109,10 +109,14 @@ String ShellManager::pipe(const String& input,
                                                 pair.first); });
 
                 if (env_var != m_env_vars.end())
+                {
+                    try
                     {
                         String value = env_var->second(name, context);
                         setenv(("kak_" + name).c_str(), value.c_str(), 1);
                     }
+                    catch (runtime_error&) {}
+                }
             }
 
             ++it;