Provide error message in Windows crypto code (fixes qpdf#286)

Thanks to github user zdenop for supplying some additional
error-handling code.

ChangeLog

@@ -1,5 +1,9 @@
 2019-06-22  Jay Berkenbilt  <[email protected]>
 
+	* Provided a more useful error message when Windows can't get
+	security context. Thanks to user zdenop for supplying some code.
+	Fixes #286.
+
 	* Favor PointerHolder over manual memory allocation in shippable
 	code where possible. Fixes #235.
 

libqpdf/SecureRandomDataProvider.cc

@@ -62,18 +62,21 @@ class WindowsCryptProvider
 #endif
             {
                 if (! CryptAcquireContext(&crypt_prov,
-                                         "Container",
-                                         NULL,
-                                         PROV_RSA_FULL,
-                                         CRYPT_NEWKEYSET))
+                                          "Container",
+                                          NULL,
+                                          PROV_RSA_FULL,
+                                          CRYPT_NEWKEYSET))
                 {
                     throw std::runtime_error(
-                        "unable to acquire crypt context with new keyset");
+                        "unable to acquire crypt context with new keyset: " +
+                        getErrorMessage());
                 }
             }
             else
             {
-                throw std::runtime_error("unable to acquire crypt context");
+                throw std::runtime_error(
+                    "unable to acquire crypt context: " +
+                    getErrorMessage());
             }
         }
     }
@@ -84,6 +87,24 @@ class WindowsCryptProvider
     }
 
     HCRYPTPROV crypt_prov;
+
+  private:
+    std::string getErrorMessage()
+    {
+        DWORD errorMessageID = ::GetLastError();
+        LPSTR messageBuffer = nullptr;
+        size_t size = FormatMessageA(
+            FORMAT_MESSAGE_ALLOCATE_BUFFER |
+            FORMAT_MESSAGE_FROM_SYSTEM |
+            FORMAT_MESSAGE_IGNORE_INSERTS, NULL, errorMessageID,
+            MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
+            reinterpret_cast<LPSTR>(&messageBuffer), 0, NULL);
+        std::string message(messageBuffer, size);
+        LocalFree(messageBuffer);
+        return ("error number " +
+                QUtil::int_to_string_base(errorMessageID, 16) +
+                ": " + message);
+    }
 };
 #endif