Add isValid field to Conversion if output track counts haven't been reached, improve error messages to be more clear (closes #148)

docs/guide/converting-media-files.md

@@ -13,6 +13,7 @@ It has the following features:
 - Video rotation
 - Video cropping
 - Video frame rate adjustment
+- Video transparency removal/preservation
 - Audio resampling
 - Audio up/downmixing
 
@@ -41,6 +42,14 @@ const output = new Output({
 });
 
 const conversion = await Conversion.init({ input, output });
+if (!conversion.isValid) {
+	// Conversion is invalid and cannot be executed without error.
+	// This field gives reasons for why tracks were discarded:
+	conversion.discardedTracks; // => DiscardedTrack[]
+
+	return;
+}
+
 await conversion.execute();
 
 // output.target.buffer contains the final file
@@ -57,7 +66,7 @@ Unconfigured, the conversion process handles all the details automatically, such
 - Copying media data whenever possible, otherwise transcoding it
 - Dropping tracks that aren't supported in the output format
 
-You should consider inspecting the [discarded tracks](#discarded-tracks) before executing a `Conversion`.
+You should consider inspecting `isValid` and the [discarded tracks](#discarded-tracks) before executing a `Conversion`.
 
 ### Monitoring progress
 
@@ -353,6 +362,8 @@ type DiscardedTrack = {
 
 Since you can inspect this list before executing a `Conversion`, this gives you the option to decide if you still want to move forward with the conversion process.
 
+If `isValid` is `false`, then the discarded tracks caused the `Conversion` to become invalid. For example, this can happen when a format requires a specific codec but that codec cannot be encoded.
+
 ---
 
 The following reasons exist: