Improve DSM diagram loading error diagnostics (#286)

When a .dsm file fails to load, the error messages were too terse
to diagnose the root cause. This adds:

- errno details on file open failures (e.g. "No such file or directory")
- A hint when the file exists with a .dsm extension appended, suggesting
  the user fix their #include directive
- Context showing which #include directive in which parent file caused
  a recursive load failure
- The configured diag_path value in the top-level load error so users
  can verify their configuration

Author: Marcel Hecko

apps/dsm/DSM.cpp

@@ -368,8 +368,10 @@ bool DSMFactory::loadDiags(AmConfigReader& cfg, DSMStateDiagramCollection* m_dia
   for (vector<string>::iterator it=
 	 diags_names.begin(); it != diags_names.end(); it++) {
     if (!m_diags->loadFile(DiagPath+*it+".dsm", *it, DiagPath, ModPath, DebugDSM, CheckDSM)) {
-      ERROR("loading %s from %s\n", 
+      ERROR("loading diagram '%s' from '%s' failed\n",
 	    it->c_str(), (DiagPath+*it+".dsm").c_str());
+      ERROR("  Check that diag_path='%s' is correct and '%s' is readable by the SEMS process\n",
+	    DiagPath.c_str(), (DiagPath+*it+".dsm").c_str());
       return false;
     }
   }

apps/dsm/DSMStateDiagramCollection.cpp

@@ -30,6 +30,7 @@
 #include <fstream>
 using std::ifstream;
 
+#include <cerrno>
 #include <dirent.h>
 
 DSMStateDiagramCollection::DSMStateDiagramCollection() {
@@ -44,8 +45,16 @@ bool DSMStateDiagramCollection::readFile(const string& filename, const string& n
 
   ifstream ifs(filename.c_str());
   if (!ifs.good()) {
-    ERROR("loading state diagram '%s'\n",
-	  filename.c_str());
+    ERROR("loading state diagram '%s': cannot open file (%s)\n",
+	  filename.c_str(), strerror(errno));
+
+    // Check if the user forgot the .dsm extension (common #include mistake)
+    string with_ext = filename + ".dsm";
+    ifstream test_ifs(with_ext.c_str());
+    if (test_ifs.good()) {
+      ERROR("  Hint: '%s' exists - did you mean to use '%s' in your #include directive?\n",
+	    with_ext.c_str(), with_ext.c_str());
+    }
     return false;
   }
 
@@ -92,6 +101,8 @@ bool DSMStateDiagramCollection::readFile(const string& filename, const string& n
 	      if (name.rfind(".dsm") != (name.size() - 4)) continue;
 	      include_dir_name = include_name + "/" + name;
 	      if (!readFile(include_dir_name, name, current_load_path, s)) {
+		ERROR("  while processing '#include %s' (directory entry '%s') in '%s'\n",
+		      include_name.c_str(), include_dir_name.c_str(), filename.c_str());
 		(void)closedir(dp);
 		return false;
 	      }
@@ -102,8 +113,11 @@ bool DSMStateDiagramCollection::readFile(const string& filename, const string& n
 	    return false;
 	  }
 	} else {
-	  if (!readFile(include_name, name, current_load_path, s))
+	  if (!readFile(include_name, name, current_load_path, s)) {
+	    ERROR("  while processing '#include %s' in '%s'\n",
+		  include_name.c_str(), filename.c_str());
 	    return false;
+	  }
 	}
 	continue;
       }