Merge branch 'bugfix/nvs_init_check_empty_pages' into 'master'

Merge branch 'bugfix/nvs_init_check_empty_pages' into 'master'

Error handling in NVS initialization

Previously nvs_flash_init worked under an assumption that there should always be at least one free page available. This is true during normal operation, but in some cases (such as when changing application partition table from a non-OTA to an OTA one), NVS partition may get truncated, which will cause empty pages to be lost.

This MR adds error checks for this condition, and updates code which calls `nvs_flash_init` to check for the return code.
For most examples, a simple `ESP_ERROR_CHECK` is added around `nvs_flash_init`. For NVS examples and the OTA example, more robust error handling is added.

This change also removes nvs_flash_init calls from examples which don't use NVS.

See merge request !582