[Qt] paymentserver: better logging of invalid certs

Before and after was tested in Windows:

before:
GUI: ReportInvalidCertificate : Payment server found
an invalid certificate:  ("Microsoft Authenticode(tm) Root Authority")
GUI: ReportInvalidCertificate : Payment server found
an invalid certificate:  ()
GUI: ReportInvalidCertificate : Payment server found
an invalid certificate:  ()
GUI: ReportInvalidCertificate : Payment server found
an invalid certificate:  ()

after:
GUI: ReportInvalidCertificate: Payment server found an
invalid certificate:  "01" ("Microsoft Authenticode(tm) Root Authority")
() ()
GUI: ReportInvalidCertificate: Payment server found an
invalid certificate:  "01" () () ("Copyright (c) 1997 Microsoft Corp.",
"Microsoft Time Stamping Service Root", "Microsoft Corporation")
GUI: ReportInvalidCertificate: Payment server found an
invalid certificate:  "4a:19:d2:38:8c:82:59:1c:a5:5d:73:5f:15:5d:dc:a3" ()
() ("NO LIABILITY ACCEPTED, (c)97 VeriSign, Inc.", "VeriSign Time Stamping
Service Root", "VeriSign, Inc.")
GUI: ReportInvalidCertificate: Payment server found an
invalid certificate:  "e4:9e:fd:f3:3a:e8:0e:cf:a5:11:3e:19:a4:24:02:32" ()
() ("Class 3 Public Primary Certification Authority")
[Qt] remove unused PaymentRequestPlus::getPKIType function
[Qt] take care of some signed/unsigned stuff in paymentrequestplus.cpp