schannel: client certificate store opening fix

1) Using CERT_STORE_OPEN_EXISTING_FLAG ( or CERT_STORE_READONLY_FLAG )
while opening certificate store would be sufficient in this scenario and
less-demanding in sense of required user credentials ( for example,
IIS_IUSRS will get "Access Denied" 0x05 error for existing CertOpenStore
call without any of flags mentioned above ),

2) as 'cert_store_name' is a DWORD, attempt to format its value like a
string ( in "Failed to open cert store" error message ) will throw null
pointer exception

3) adding GetLastError(), in my opinion, will make error message more
useful.

Bug: https://curl.haxx.se/mail/lib-2018-08/0198.html

Closes #2909

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