Improve documentation around error messages

[tompizmor]

The rationale behind this is to let users know what error messages mean and let them know that the rest of the charts will be synced and only the charts with the error message will be skipped.

One of the examples could be a missing dependency for a specific chart. Chart-syncer will throw an error at the end of the execution and will skip syncing that specific chart but the rest of them will be properly synced