unknown database tag exception introduced

DbService.cs

@@ -28,6 +28,9 @@ namespace com.krohne.genericdatabaseapiservice.Services {
         public string Name { get; set; }
     }
 
+    public class DbInfoServiceException : Exception {}
+    public class UnknownDatabaseTagException: DbInfoServiceException {}
+
     public interface IDbInfoService {
         DbInfoObject GetInfoByTag(string tag);
         string GetInfoStringByTag(string tag);
@@ -44,16 +47,20 @@ namespace com.krohne.genericdatabaseapiservice.Services {
         }
 
         public DbInfoObject GetInfoByTag(string tag) {
-            return DbInfos[tag];
+            try {
+                return DbInfos[tag];
+            } catch (KeyNotFoundException) {
+                throw new UnknownDatabaseTagException();
+            }
         }
 
         public string GetInfoStringByTag(string tag) {
             return String.Format(
                 "Server={0};User ID={1};Password={2};Database={3}",
-                DbInfos[tag].Host,
-                DbInfos[tag].User,
-                DbInfos[tag].Password,
-                DbInfos[tag].Name);
+                GetInfoByTag(tag).Host,
+                GetInfoByTag(tag).User,
+                GetInfoByTag(tag).Password,
+                GetInfoByTag(tag).Name);
         }
     }