NewHistogramReporter swallows write errors (return nil instead of return err)

Bug Description

In lib/reporters.go, the NewHistogramReporter function has a typo where a write error is silently swallowed:

for i, count := range h.Counts {
    ratio := float64(count) / float64(h.Total)
    lo, hi := h.Buckets.Nth(i)
    pad := strings.Repeat("#", int(ratio*75))
    _, err = fmt.Fprintf(tw, "[%s,\t%s]\t%d\t%.2f%%\t%s\n", lo, hi, count, ratio*100, pad)
    if err != nil {
        return nil  // BUG: should be `return err`
    }
}

The header write at line 38 correctly returns err, but the bucket write at line 47 returns nil, swallowing the error.

Impact

Write errors (e.g., broken pipe, disk full) during histogram bucket output are silently discarded. The caller receives no indication that the report was incomplete.

Fix

Change return nil to return err.

PR forthcoming.