Making error message user-friendly (suggesting fix) and increasing recommended maximum upload size to 8 MB

# Executive summary

1. Error message for maximum upload size is not hinting exact origin of problem nor how to fix the issue.
2. Recommended maximum upload size is 4 MB, which is smaller than most major components.
3. Error messages for upload size and post size wrongly indicate 2 MB instead of 4 MB now and are fixed to 8 MB with this PR too.

This PR is independent of https://github.com/joomla/joomla-cms/pull/4209 , but is a useful addition.

## Backwards compatibility

No backwards compatibility issues are introduced.

## Translation impact

No translation strings are added or deleted, but 2 strings are updated to be more precise.

## Testing instructions

Apply https://github.com/joomla/joomla-cms/pull/4209

Test 1: Make sure that your server's php.ini has a max_upload_size of 1M. Try installing an extension bigger than that, e.g. Admin Tools Core, K2, Community Builder etc. You get a generic upload error. Apply the PR and retry. You now get an error telling you that the upload failed and that your maximum upload limit is too small.

Test 2: Go to Extensions / Extensions Manager and click Warnings on side-bar. You should see the updated message and limit too.