Add some additional check to NavCache opening.

Hoping to capture why opening the DB read-write fails in some cases
on Windows.

src/Navaids/NavDataCache.cxx

@@ -78,6 +78,7 @@
 #include <Airports/parking.hxx>
 #include <Airports/gnnode.hxx>
 #include "CacheSchema.h"
+#include <GUI/MessageBox.hxx>
 
 using std::string;
 
@@ -263,10 +264,17 @@ public:
 
       int openFlags = readOnly ? SQLITE_OPEN_READONLY :
         SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE;
-      // see http://code.google.com/p/flightgear-bugs/issues/detail?id=1055
-      // for the UTF8 / path logic here
       std::string pathUtf8 = path.utf8Str();
-    sqlite3_open_v2(pathUtf8.c_str(), &db, openFlags, NULL);
+      int result = sqlite3_open_v2(pathUtf8.c_str(), &db, openFlags, NULL);
+      if (result == SQLITE_MISUSE) {
+          // docs state sqlite3_errmsg may not work in this case
+          SG_LOG(SG_NAVCACHE, SG_WARN, "Failed to open DB at " << path << ": misuse of Sqlite API");
+          throw sg_exception("Navcache failed to open: Sqlite API misuse");
+      } else if (result != SQLITE_OK) {
+          std::string errMsg = sqlite3_errmsg(db);
+          SG_LOG(SG_NAVCACHE, SG_WARN, "Failed to open DB at " << path << " with error:\n\t" << errMsg);
+          throw sg_exception("Navcache failed to open:" + errMsg);
+      }
 
     sqlite3_stmt_ptr checkTables =
       prepare("SELECT count(*) FROM sqlite_master WHERE name='properties'");
@@ -1207,11 +1215,25 @@ NavDataCache::NavDataCache()
         } catch (sg_exception& e) {
             SG_LOG(SG_NAVCACHE, t == 0 ? SG_WARN : SG_ALERT, "NavCache: init failed:" << e.what()
                    << " (attempt " << t << ")");
+
+            if (t == (MAX_TRIES - 1)) {
+                // final attempt still failed, we are busted
+                flightgear::fatalMessageBox("Unable to open navigation cache",
+                                            std::string("The navigation data cache could not be opened:") + e.getMessage(),
+                                            e.getOrigin());
+            }
+
             d.reset();
 
             // only wipe the existing if not readonly
             if (!fgGetBool("/sim/fghome-readonly", false)) {
-            homePath.remove();
+                bool ok = homePath.remove();
+                if (!ok) {
+                    SG_LOG(SG_NAVCACHE, SG_ALERT, "NavCache: failed to remove previous cache file");
+                    flightgear::fatalMessageBox("Unable to re-create navigation cache",
+                                                "Attempting to remove the old cache failed.",
+                                                "Location: "  + homePath.utf8Str());
+                }
             }
         }
     } // of retry loop