layers: Change pNext validation error to warning

Change the severity level of the parameter_validation layer's pNext
parameter validation messages from error to warning.  Also add an
explanation that the warning is derived from the Valid Usage
language from version X of the Vulkan spec/header, which may be out
of date:

This warning is based on the Valid Usage documentation for version %d
of the Vulkan header.  It is possible that you are using a struct
from a private extension or an extension that was added to a later
version of the Vulkan header, in which case your use of pNext is
perfectly valid but is not guaranteed to work correctly with
validation enabled

Issues-Addressed: LunarXchange 560
Change-Id: I97f55cba7538506d246e45ef603a86db9ad1adcb
diff --git a/tests/layer_validation_tests.cpp b/tests/layer_validation_tests.cpp
index d41ad96..e91a3d1 100644
--- a/tests/layer_validation_tests.cpp
+++ b/tests/layer_validation_tests.cpp
@@ -972,7 +972,7 @@
     ASSERT_NO_FATAL_FAILURE(InitState());
 
     m_errorMonitor->SetDesiredFailureMsg(
-        VK_DEBUG_REPORT_ERROR_BIT_EXT,
+        VK_DEBUG_REPORT_WARNING_BIT_EXT,
         "value of pCreateInfo->pNext must be NULL");
     // Set VkMemoryAllocateInfo::pNext to a non-NULL value, when pNext must be
     // NULL.
@@ -989,7 +989,7 @@
     m_errorMonitor->VerifyFound();
 
     m_errorMonitor->SetDesiredFailureMsg(
-        VK_DEBUG_REPORT_ERROR_BIT_EXT,
+        VK_DEBUG_REPORT_WARNING_BIT_EXT,
         " chain includes a structure with unexpected VkStructureType ");
     // Set VkMemoryAllocateInfo::pNext to a non-NULL value, but use
     // a function that has allowed pNext structure types and specify
@@ -1004,7 +1004,7 @@
     m_errorMonitor->VerifyFound();
 
     m_errorMonitor->SetDesiredFailureMsg(
-        VK_DEBUG_REPORT_ERROR_BIT_EXT,
+        VK_DEBUG_REPORT_WARNING_BIT_EXT,
         " chain includes a structure with unexpected VkStructureType ");
     // Set VkGraphicsPipelineCreateInfo::VkPipelineRasterizationStateCreateInfo::pNext to an invalid structure, when pNext is allowed to be a non-NULL value
     // Expected to trigger an error with