Refactor CSV Import Errors

[milafrerichs]

• use dictionary for csv reading as well (https://github.com/OpenUpSA/wazimap-ng/blob/staging/wazimap_ng/points/admin/coordinate_file_admin.py#L65)
• show error detail in Admin view and not the column name (https://github.com/openupsa/wazimap-ng/blob/staging/wazimap_ng/datasets/templates/custom/variable_task_errors.html#L6)
• not record 2 errors when the header is missing
• notification should mention that there were errors

related to this PR: https://github.com/OpenUpSA/wazimap-ng/pull/123/files

[BarisSari]

I'm thinking to take this issue but it seems like it's related to #128. Should I wait for that PR to be merged first, @milafrerichs and @goyal1092 ?