Confusing error message when SQLAlchemy is not installed
Trying to use read_sql_table on an instance without SQLAlchemy installed gives a confusing error message:

> NotImplementedError: read_sql_table only supported for SQLAlchemy connectable.

The issue is that `_is_sqlalchemy_connectable` returns false if SQLAlchemy is not installed. My suggestion is to raise an error stating that SQLAlchemy is required by trying to import it directly in read_sql_table or extending the error message to say that SQLAlchemy is required. I don't know the internals and what might go wrong, but one way would be to not catch the ImportError...

https://github.com/pandas-dev/pandas/blob/49c779118c2d473c60565d9010e119bb62d06f74/pandas/io/sql.py#L308-L311