Better error messages for some S3 functions

[stavrospapadopoulos]

For example S3::is_bucket just returns false, even when S3 is not supported.

[ihnorton]

Should is_bucket simply return false for a local path? Currently it raises an error (whether or not the path exists):

>>> tdb_ctx = tiledb.Ctx()
... vfs = tiledb.VFS(tdb_ctx.config())
>>> vfs.is_bucket("/Users/inorton/data/seg1")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "tiledb/libtiledb.pyx", line 5319, in tiledb.libtiledb.VFS.is_bucket
  File "tiledb/libtiledb.pyx", line 248, in tiledb.libtiledb._raise_ctx_err
  File "tiledb/libtiledb.pyx", line 233, in tiledb.libtiledb._raise_tiledb_error
tiledb.libtiledb.TileDBError: [TileDB::VFS] Error: Unsupported URI scheme: file:///Users/inorton/data/seg1

[TileDB::VFS] Error: Unsupported URI scheme: file:///Users/inorton/data/seg1