Add -v CLI flag that prints all paper parsing errors

[bzz]

Right now on parsing the papers we only count all errors, silently skipping failures and thus hiding their root cause.

scholar-alert-digest/papers/papers.go

Lines 46 to 48 in 7d2e4de

	// Stats is a number of counters \w stats on paper extraction from gmail messages.
	type Stats struct {
	Msgs, Titles, Errs int

This leads to difficulties reproducing bugs like #76 as one needs to modify source code (by adding log.Print(err) to

scholar-alert-digest/papers/papers.go

Lines 83 to 84 in 7d2e4de

	if err != nil {
	st.Errs++

) for debugging.

Instead, it would be nice to have:

• a -v flag that would print all the errors
• accumulated (e.g as map[Subject][]error) through paper parsing, as part of Stats

That would simplify the debugging, as users would be able to identify the offender emails and attach the specific HTMLs that causes errors.