Improve error messages

[bhou]

In some cases, java exceptions are displayed directly in error message notification. Need to provide more user-friendly messages.

[millboh]

We need to display the amount of skipped lines due to errors and the n first errors. An advanced parameters in all steps should be "max_errors". Technically speaking, we already have an error_handler in most steps. We can refactor it in more abstract classes to handle errors with this advanced setting. By default i suggest max_errors should set to 0 .

[bhou]

@millboh Putting a max_errors for each step sounds like a bit overkill. We can add a global settings max_errors, and pass it in dag request. For the default value, 0 seems too strict, and it will break some existing projects.

[millboh]

@bhou I suggested max_errors to be step specific because it actually depends on the volume of data processed. For an excel source 0 might fit but if you process TB of data you'll need a different max_error value.