traverser: clear error messages before traversal operations

Recent debugging revealed that the traverser error message wasn't
getting cleared after some traversal operations, leading to spurious or
error logging that can be unrelated to the error cause.

Start each public traverser API call with clear_err_message () to ensure
error messages are correctly attributed to the cause.

resource/traversers/dfu.cpp

@@ -312,6 +312,9 @@ bool dfu_traverser_t::is_initialized () const
 
 int dfu_traverser_t::initialize ()
 {
+    // Clear the error message to disambiguate errors
+    clear_err_message ();
+
     int rc = 0;
     vtx_t root;
     if (!get_graph () || !get_graph_db () || !get_match_cb ()) {
@@ -350,6 +353,9 @@ int dfu_traverser_t::run (Jobspec::Jobspec &jobspec,
                           int64_t jobid,
                           int64_t *at)
 {
+    // Clear the error message to disambiguate errors
+    clear_err_message ();
+
     subsystem_t dom = get_match_cb ()->dom_subsystem ();
     graph_duration_t graph_duration = get_graph_db ()->metadata.graph_duration;
     if (!get_graph () || !get_graph_db ()
@@ -413,6 +419,9 @@ int dfu_traverser_t::run (const std::string &str,
                           int64_t at,
                           uint64_t duration)
 {
+    // Clear the error message to disambiguate errors
+    clear_err_message ();
+
     if (!get_match_cb () || !get_graph () || !get_graph_db () || !reader || at < 0) {
         errno = EINVAL;
         return -1;
@@ -435,11 +444,16 @@ int dfu_traverser_t::run (const std::string &str,
 
 int dfu_traverser_t::find (std::shared_ptr<match_writers_t> &writers, const std::string &criteria)
 {
+    // Clear the error message to disambiguate errors
+    clear_err_message ();
     return detail::dfu_impl_t::find (writers, criteria);
 }
 
 int dfu_traverser_t::remove (int64_t jobid)
 {
+    // Clear the error message to disambiguate errors
+    clear_err_message ();
+
     subsystem_t dom = get_match_cb ()->dom_subsystem ();
     if (!get_graph () || !get_graph_db ()
         || get_graph_db ()->metadata.roots.find (dom) == get_graph_db ()->metadata.roots.end ()
@@ -457,6 +471,9 @@ int dfu_traverser_t::remove (const std::string &R_to_cancel,
                              int64_t jobid,
                              bool &full_cancel)
 {
+    // Clear the error message to disambiguate errors
+    clear_err_message ();
+
     subsystem_t dom = get_match_cb ()->dom_subsystem ();
     if (!get_graph () || !get_graph_db ()
         || get_graph_db ()->metadata.roots.find (dom) == get_graph_db ()->metadata.roots.end ()
@@ -471,11 +488,15 @@ int dfu_traverser_t::remove (const std::string &R_to_cancel,
 
 int dfu_traverser_t::mark (const std::string &root_path, resource_pool_t::status_t status)
 {
+    // Clear the error message to disambiguate errors
+    clear_err_message ();
     return detail::dfu_impl_t::mark (root_path, status);
 }
 
 int dfu_traverser_t::mark (std::set<int64_t> &ranks, resource_pool_t::status_t status)
 {
+    // Clear the error message to disambiguate errors
+    clear_err_message ();
     return detail::dfu_impl_t::mark (ranks, status);
 }