[commands] Exceptions raised while invocation raise CommandInvokeError.

This change should make it a bit more intuitive to get the original
exception without having the gotcha of checking ``isinstance`` inside
the error handler.

discord/ext/commands/core.py

@@ -47,7 +47,7 @@ def inject_context(ctx, coro):
         try:
             ret = yield from coro(*args, **kwargs)
         except Exception as e:
-            raise CommandError("Exception raised while executing command") from e
+            raise CommandInvokeError(e) from e
         return ret
     return wrapped