Signature Validation : Change Error Thrown

This commit changes the error reported when the signature
requirements are not met. Instead of reporting "InternalError",
the newly defined InvalidSignature error is thrown in its place.
This allows there to be a more specific error shown.

Tested:
Before:
Apr 08 19:04:44 witherspoon phosphor-image-updater[416]: Error occurred during image validation
Apr 08 19:04:44 witherspoon phosphor-image-updater[416]: The operation failed internally.

After:
Jan 01 00:11:10 witherspoon phosphor-image-updater[403]: Signature Validation failed for image version.

Signed-off-by: Zami Seck <zimzam17@gmail.com>
Change-Id: Ia92aba70ea4b346b1473b557a5adeeea7a3cae3d
diff --git a/activation.cpp b/activation.cpp
index 8e71d8c..e057814 100644
--- a/activation.cpp
+++ b/activation.cpp
@@ -94,7 +94,9 @@
         fs::path uploadDir(IMG_UPLOAD_DIR);
         if (!verifySignature(uploadDir / versionId, SIGNED_IMAGE_CONF_PATH))
         {
-            onVerifyFailed();
+            using InvalidSignatureErr = sdbusplus::xyz::openbmc_project::
+                Software::Version::Error::InvalidSignature;
+            report<InvalidSignatureErr>();
             // Stop the activation process, if fieldMode is enabled.
             if (parent.control::FieldMode::fieldModeEnabled())
             {
@@ -108,7 +110,6 @@
 
         if (!minimum_ship_level::verify(versionStr))
         {
-            using namespace phosphor::logging;
             using IncompatibleErr = sdbusplus::xyz::openbmc_project::Software::
                 Version::Error::Incompatible;
             using Incompatible =
@@ -342,12 +343,6 @@
 
     return signature.verify();
 }
-
-void Activation::onVerifyFailed()
-{
-    error("Error occurred during image validation");
-    report<InternalFailure>();
-}
 #endif
 
 void ActivationBlocksTransition::enableRebootGuard()