More descriptive error message

Make the error message of package verification more descriptive
to help debugging.

Before the change, the error message is:
"Package Verification Result".
After the change, the error message will be like:
"some error message".

* Add VerificationParams.mErrorMessage to remember the error
  message as well as the error code.
* All changes to mRet/mErrorMessage must be via setReturnCode().

Bug: 181092466
Test: atest StagedInstallTest
Test: atest GtsSecurityHostTestCases
Test: atest CtsAtomicInstallTestCases
Test: atest GkiInstallTest
Change-Id: I3e0135e69cf6e745b0ccb7cbae3c9b40ef556c1e