apps: cam: Print an error when outputting DNG and DNG support is missing

When DNG support is missing, the cam application ignores the .dng suffix
of the file pattern and writes raw binary data instead, without
notifying the user. This leads to confusion. Fix it by printing an error
message.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Milan Zamazal <mzamazal@redhat.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>

src/apps/cam/camera_session.cpp

@@ -256,11 +256,16 @@ int CameraSession::start()
 #endif
 
 	if (options_.isSet(OptFile)) {
-		if (!options_[OptFile].toString().empty())
-			sink_ = std::make_unique<FileSink>(camera_.get(), streamNames_,
-							   options_[OptFile]);
-		else
-			sink_ = std::make_unique<FileSink>(camera_.get(), streamNames_);
+		std::unique_ptr<FileSink> sink =
+			std::make_unique<FileSink>(camera_.get(), streamNames_);
+
+		if (!options_[OptFile].toString().empty()) {
+			ret = sink->setFilePattern(options_[OptFile]);
+			if (ret)
+				return ret;
+		}
+
+		sink_ = std::move(sink);
 	}
 
 	if (sink_) {