Make error param non-optional when setting local/remote content.

This is a slight refactoring while doing some other changes, so not
strictly necessary, but the error param is always supplied in practice
so it made sense to update the tests to reflect that, test that error
values are reported in (at least) some cases and remove the additional
code that checks for whether or not error information is requested.

Bug: none
Change-Id: Ia5739a18ea2beb6970eabf9d809c24dfa43466b1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/244097
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35632}

pc/channel_interface.h

@@ -51,10 +51,10 @@ class ChannelInterface {
   // Channel control
   virtual bool SetLocalContent(const MediaContentDescription* content,
                                webrtc::SdpType type,
-                               std::string* error_desc) = 0;
+                               std::string& error_desc) = 0;
   virtual bool SetRemoteContent(const MediaContentDescription* content,
                                 webrtc::SdpType type,
-                                std::string* error_desc) = 0;
+                                std::string& error_desc) = 0;
   virtual bool SetPayloadTypeDemuxingEnabled(bool enabled) = 0;
 
   // Access to the local and remote streams that were set on the channel.