tweak error behavior for redeclared uniforms for vulkan-relaxed

avoids nullptr crash from reading memberType's name, and more user
friendly error message.

Test/vk.relaxed.errorcheck.frag

@@ -7,10 +7,13 @@ out vec4 o;
 // default uniforms will be gathered into a uniform block
 uniform vec4 a;     // declared in both stages with different types
 
+uniform float test; // declared twice in this compilation unit
+uniform vec2 test;
+
 vec4 foo() {
-    return a;
+    return a + vec4(test);
 }
 
 void main() {
     o = io + foo();
-}
+}