Started to happen since 5.4.0b9.
Reproduced on 5.4.0b9 and b10.

Editor from 5.4.0b9 has stopped temporary change specified platform in BuildPipeline, when building AssetBundles. If at some point project has been set to WebPlayer platform, and later on, that platform was removed or wasn't installed with a new Unity version, building AssetBundles console throws an error message, which doesn't tell anything useful to user:

Error:
IsBuildTargetSupported(targetPlatform)
UnityEditor.BuildPipeline:BuildAssetBundles(String, BuildAssetBundleOptions, BuildTarget)
CreateAssetBundles:BuildAllAssetBundles() (at Assets/BundleCompiler.cs:10)

This message means, that that selected platform in project settings (in this case WebPlayer) is not installed with a current Unity version.

Workaround:
To avoid the error, in "Build Settings", user has to switch to any other installed platform (for example: if standalone was installed, then manual switch to this platform has to be performed.).

Repro steps:
1. Open project "TestCase.zip". Editor should not have WebPlayer platform installed.
2. Build AssetBindles pressing menu bar "Assets -> Build AssetBundles".
3. Error is showed in the console. Observe in "Build Settings" that none of the platforms are actively selected.

Expected results: get a readable error message.