MAGETWO-43906: Long database prefix length breaks setup

- changes according to CR

Author: Ivan Gavryshko

setup/src/Magento/Setup/Test/Unit/Validator/DbValidatorTest.php

@@ -89,7 +89,7 @@ public function testCheckDatabaseTablePrefixWrongFormat()
 
     /**
      * @expectedException \InvalidArgumentException
-     * @expectedExceptionMessage Table prefix length can't be more then
+     * @expectedExceptionMessage Table prefix length can't be more than
      */
     public function testCheckDatabaseTablePrefixWrongLength()
     {

setup/src/Magento/Setup/Validator/DbValidator.php

@@ -16,6 +16,9 @@
 class DbValidator
 {
 
+    /**
+     * Db prefix max length
+     */
     const DB_PREFIX_LENGTH = 10;
 
     /**
@@ -47,12 +50,15 @@ public function checkDatabaseTablePrefix($prefix)
         //The table prefix should contain only letters (a-z), numbers (0-9) or underscores (_);
         // the first character should be a letter.
         if ($prefix !== '' && !preg_match('/^([a-zA-Z])([[:alnum:]_]+)$/', $prefix)) {
-            throw new \InvalidArgumentException('Please correct the table prefix format.');
+            throw new \InvalidArgumentException(
+                'Please correct the table prefix format, should contain only numbers, letters or underscores.'
+                .' The first character should be a letter.'
+            );
         }
 
         if (strlen($prefix) > self::DB_PREFIX_LENGTH) {
             throw new \InvalidArgumentException(
-                'Table prefix length can\'t be more then ' . self::DB_PREFIX_LENGTH . ' characters.'
+                'Table prefix length can\'t be more than ' . self::DB_PREFIX_LENGTH . ' characters.'
             );
         }