Fix "the dialect is not supported" error

This makes it so that, if the provided dialect is a supported dialect (eg. postgres) and requiring the dialect throws an error (eg. the 'pg' package not being installed), the actual underlying error and its accompanying stack trace are thrown to the caller, rather than stringifying the error (losing the stack) and reporting the wrong error.

lib/sequelize.js

@@ -188,7 +188,6 @@ var Sequelize = function(database, username, password, options) {
     dialectOptions: this.options.dialectOptions
     dialectOptions: this.options.dialectOptions
   };
   };
 
 
-  try {
   var Dialect;
   var Dialect;
   // Requiring the dialect in a switch-case to keep the
   // Requiring the dialect in a switch-case to keep the
   // require calls static. (Browserify fix)
   // require calls static. (Browserify fix)
@@ -208,11 +207,10 @@ var Sequelize = function(database, username, password, options) {
     case 'sqlite':
     case 'sqlite':
       Dialect = require('./dialects/sqlite');
       Dialect = require('./dialects/sqlite');
       break;
       break;
+    default:
+      throw new Error('The dialect ' + this.getDialect() + ' is not supported. Supported dialects: mariadb, mssql, mysql, postgres, and sqlite.');
   }
   }
   this.dialect = new Dialect(this);
   this.dialect = new Dialect(this);
-  } catch (err) {
-    throw new Error('The dialect ' + this.getDialect() + ' is not supported. Supported dialects: mariadb, mssql, mysql, postgres, and sqlite. ('+err+')');
-  }
 
 
   this.dialect.QueryGenerator.typeValidation = options.typeValidation;
   this.dialect.QueryGenerator.typeValidation = options.typeValidation;