Add blockquotes for warning messages in range [PRJ0018, PRJ0049]

Author: Rageking8

docs/error-messages/tool-errors/project-build-warning-prj0018.md

@@ -8,7 +8,7 @@ ms.assetid: 1a3d9e40-6a35-4a74-b6ba-8079f7a82217
 ---
 # Project Build Warning PRJ0018
 
-The following environment variables were not found:
+> The following environment variables were not found:
 
 An environment variable is not defined. This error lists the environment variables that were not defined.
 

docs/error-messages/tool-errors/project-build-warning-prj0029.md

@@ -8,7 +8,7 @@ ms.assetid: f02c09c6-09f3-4d44-8cd4-9a25336be1ea
 ---
 # Project Build Warning PRJ0029
 
-The 'Outputs' property for the project-level custom build step is not set. The custom build step will be skipped.
+> The 'Outputs' property for the project-level custom build step is not set. The custom build step will be skipped.
 
 A custom build step was not executed because no output was specified.
 

docs/error-messages/tool-errors/project-build-warning-prj0041.md

@@ -8,7 +8,7 @@ ms.assetid: dc9f4cf9-6bd5-4222-89e8-7802a59bb96b
 ---
 # Project Build Warning PRJ0041
 
-Cannot find missing dependency 'dependency' for file 'file'. Your project may still build, but may continue to appear out of date until this file is found.
+> Cannot find missing dependency 'dependency' for file 'file'. Your project may still build, but may continue to appear out of date until this file is found.
 
 A file (resource file or .idl/.odl file, for example, contained an include statement that the project system could not resolve.
 

docs/error-messages/tool-errors/project-build-warning-prj0049.md

@@ -7,7 +7,7 @@ ms.assetid: 8b38afa1-e080-4efd-ae89-776cfd044413
 ---
 # Project Build Warning PRJ0049
 
-Referenced target '\<Reference>' requires .NET Framework \<MinFrameworkVersion> and will fail to run on this project's target framework
+> Referenced target '\<Reference>' requires .NET Framework \<MinFrameworkVersion> and will fail to run on this project's target framework
 
 Applications created by using Visual Studio 2008 can specify which version of the .NET Framework they should target. If you add a reference to an assembly or project that depends on a version of the .NET Framework that is later than the targeted version, you will get this warning at compile time.