Changes to NewObject() function signature.

Hi,

I tried to send an email to https://groups.google.com/forum/#!forum/rocksdb, as this is more of a discussion/question but my email client could not resolve the recipient, so posting here.

**Issue:**
```
db_bench, db_stress and other apps crash when a malformed/erroneous string is passed to --fs_uri or –env_uri option.
============Example=============
./db_bench --benchmarks=fillrandom --num=10000 --fs_uri=zenfs://xyz:nvm0n
Received signal 11 (Segmentation fault)
Segmentation fault
================================
```

**Rootcause**:
The factory function called by `NewObject()` do not allow for returning error codes from underlying filesystems and environments, it always returns a object pointer, if the pointer is null then it is returned as `Status::NotSupported`
Since the `ConfigOption.ignore_unsupported_options` is true by default, underlying layers returns Status::OK instead of the error.

In case of ZenFS plugin, it identifies the error in the string, updates the errormsg and returns a null pointer. But it gets translated to
Status::NotSupported("errmsg"), and since the "ignore_unsupported_options" in ConfigOptions is true by default, the error is
is ignored and it gets converted to Status::OK in customizable_util.h L68, and results in a crash when the apps(db-bench)
subsequently try to make the filesystem calls.

**Ideal Solution:**
Change the signature of the factory function which is called from NewObject, so that it can return error codes (not just a null or a valid pointer) and these error codes could be propagated to upper layers.
But this may break things for other users. Would such a change PR be acceptable ?
So what are your thoughts on this ? Is there any other way to fix it?


Other Probable Quick Fixes and side-effects:

1. Setting "ignore_unsupported_options" to false in config_options inside CreateFromUri(), but this is not possible as config_options is passed as a "const", so it is immutable, (so change the prototype of CreateFromUri() and deal with it in all places CreateFromUri() is called)
2. Setting “ignore_unsupported_options = false” before calling CreateFromUri(), but that means anyone calling CreateFromUri() in future should also be aware of this, which will be unreliable. Setting the "ignore_unsupported_options" to false by default is also a no-go as it breaks a lot of other tests.
3. Create a new ConfigOptions variable in CreateFromUri(), set “ignore_unsupported_options = false” and pass it on to CreateFromEnv(), so the error is reported back to the
application and the failure is graceful and apps don't crash.

I have a patch ready for option 3.

Thanks
Aravind

Start with customizable_util.h at line 68 and trace CreateFromUri() into CreateFromEnv(), focusing on how ConfigOptions.ignore_unsupported_options handles factory failures. Review the existing patch or linked pull request, then verify that malformed --fs_uri and --env_uri values produce an application error rather than a later crash.

Written by the indexing model from the issue text.