atrace: don't hard-fail if one of the categories doesn't exist

Before this CL:
`atrace --async_start sched wm foobar` would fail, without enabling
sched and wm if one of the categories (foobar) did not exist.

After this CL:
It prints an error but enables the other categories.

Note that the "error enabling tracing category" fprintf was
also redundant because setCategoryEnable() has already its
own fprintf.

This is to address bugs where people copy/paste a config from a new
device onto an old device, and then the whole userspace tracing fails
because one of the many atrace categories doesn't exist.

This makes it consistent with the behavior of ftrace, where we keep
going if some of the ftrace events doesn't exist.

Bug: 171085599
Test: manual. `perfetto -t 1s bionic am foobar -o trace` outputs a
      trace which contains wm and am events.
Change-Id: I5e807b6466a2cefc5a41ef53b197984a8308060c