Improve build script robustness.

Add error check as long as possible.
Note that install script does not have error check because it cannot stop build process even if it returns non-zero exit code.

Author: yfakariya

appveyor-debug.yml

@@ -25,11 +25,15 @@ build_script:
 
     ./Build.ps1
 
+    if ( $LastExitCode -ne 0 )
+    {
+        Write-Error "Failed to build."
+        exit 1
+    }
+
     cd ..
 test_script:
 - cmd: >-
     cd ./build
 
     ./RunUnitTests.cmd
-
-    cd ..

appveyor-release.yml

@@ -28,12 +28,36 @@ build_script:
 
     ./Build.ps1
 
+    if ( $LastExitCode -ne 0 )
+    {
+        Write-Error "Failed to build."
+        exit 1
+    }
+
     appveyor PushArtifact "../dist/MsgPack.Cli.${env:PackageVersion}.nupkg"
 
+    if ( $LastExitCode -ne 0 )
+    {
+        Write-Error "Failed to publish nupkg."
+        exit 1
+    }
+
     appveyor PushArtifact "../dist/MsgPack.Cli.${env:PackageVersion}.symbols.nupkg"
 
+    if ( $LastExitCode -ne 0 )
+    {
+        Write-Error "Failed to publish symbol nupkg."
+        exit 1
+    }
+
     appveyor PushArtifact "../dist/MsgPack.Cli.${env:PackageVersion}.zip"
 
+    if ( $LastExitCode -ne 0 )
+    {
+        Write-Error "Failed to publish zip."
+        exit 1
+    }
+
     cd ..
 deploy:
 - provider: Environment