Progress: Add and MustAdd methods

Nobody likes sudden panics,
thus add opportunity to handle error.

progress.go

@@ -128,13 +128,26 @@ func (p *Progress) AddSpinner(total int64, options ...BarOption) *Bar {
 
 // New creates a bar by calling `Build` method on provided `BarFillerBuilder`.
 func (p *Progress) New(total int64, builder BarFillerBuilder, options ...BarOption) *Bar {
-	return p.AddFiller(total, builder.Build(), options...)
+	return p.MustAdd(total, builder.Build(), options...)
 }
 
-// AddFiller creates a bar which renders itself by provided filler.
-// If `total <= 0` triggering complete event by increment methods is disabled.
-// Panics if *Progress instance is done, i.e. called after (*Progress).Wait().
-func (p *Progress) AddFiller(total int64, filler BarFiller, options ...BarOption) *Bar {
+// MustAdd creates a bar which renders itself by provided BarFiller.
+// If `total <= 0` triggering complete event by increment methods is
+// disabled. Panics if *Progress instance is done, i.e. called after
+// (*Progress).Wait().
+func (p *Progress) MustAdd(total int64, filler BarFiller, options ...BarOption) *Bar {
+	bar, err := p.Add(total, filler, options...)
+	if err != nil {
+		panic(err)
+	}
+	return bar
+}
+
+// Add creates a bar which renders itself by provided BarFiller.
+// If `total <= 0` triggering complete event by increment methods
+// is disabled. If *Progress instance is done, i.e. called after
+// (*Progress).Wait(), return err == DoneError.
+func (p *Progress) Add(total int64, filler BarFiller, options ...BarOption) (*Bar, error) {
 	if filler == nil {
 		filler = NopStyle().Build()
 	}
@@ -168,9 +181,9 @@ func (p *Progress) AddFiller(total int64, filler BarFiller, options ...BarOption
 				bs.shutdownListeners = append(bs.shutdownListeners, d)
 			}
 		})
-		return bar
+		return bar, nil
 	case <-p.done:
-		panic(DoneError)
+		return nil, DoneError
 	}
 }