Load improvements

[volcan01010]

This pull request makes a number of small improvements to close multiple issues.

To test

Most of the changes are covered by tests. To see them manually:

Not covered by tests

• [x] Uninstall cx_Oracle and run python etlhelper/setup_oracle_client.py. There should be a helpful error message. Closes #101
• [x] Try to connect to a database using an incorrect password. The error message should contain the DbParams and no clear-text password. There is no ticket for this.

Covered by tests

• [x] View README section and extra tests for generate_insert_sql. Closes #119
• [x] Run load command (e.g. from recipe example in README) and note that the number of rows processed and failed is returned. Closes #114
• [x] Run load command with empty array and note lack of error. Closes #120

[ximenesuk]

This looks good locally. I pushed a very minor flake8 fix to satisfy the developer tests. One of the Python builds is failing. Is this a pull-stopper?

[volcan01010]

I got this working now. There was something wrong with the pip install process in the Travis build for Python 3.7. I upgraded some of the install dependencies and now it works. Will merge. Thanks for review.