pull: relay along errors encountered

Fixes #734.

Capture errors after parallelized pulls.
Also avoid shadowing errors on result waiting.

The changed code was legacy of an oversight after the old days
in which we didn't send back exit codes to the outside world.
After exit codes were added to drive, the pull code was never touched.
Even still though, we had logging for errors and users could see their
errors.

Exhibits:
* Unsuccessful pull
```shell
 drive pull --export pdf,rtf t1; echo $?
Resolving...
$ drive pull --export pdf,rtf t1; echo $?
Resolving...
M /test-docs/t1
Modification count 1
yroceed with the changes? [Y/n]: –
/test-docs/t1 err: download: failed for url "". StatusCode: 400
download: failed for url "". StatusCode: 400
255
```

* Successful pull
```shell
$ drive pull --export pdf,rtf t1; echo $?
Resolving...
M /test-docs/t1
Modification count 1
Proceed with the changes? [Y/n]:y
Exported '/Users/emmanuelodeke/emm.odeke@gmail.com/test-docs/t1' to '/Users/emmanuelodeke/emm.odeke@gmail.com/test-docs/t1_exports/t1.rtf'
Exported '/Users/emmanuelodeke/emm.odeke@gmail.com/test-docs/t1' to '/Users/emmanuelodeke/emm.odeke@gmail.com/test-docs/t1_exports/t1.pdf'
0
```

src/pull.go

@@ -449,9 +449,10 @@ func (g *Commands) playPullChanges(cl []*Change, exports []string, opMap *map[Op
 
 	results := semalim.Run(jobsChan, uint64(n))
 	for result := range results {
-		res, err := result.Value(), result.Err()
-		if err != nil {
-			g.log.LogErrf("pull: %s err: %v\n", res, err)
+		res, rErr := result.Value(), result.Err()
+		if rErr != nil {
+			msg := fmt.Sprintf("%v err: %v\n", res, rErr)
+			err = reComposeError(err, msg)
 		}
 	}
 
@@ -602,13 +603,17 @@ func (g *Commands) makeExportsDir(segments ...string) string {
 }
 
 func (g *Commands) export(f *File, destAbsPath string, exports []string) (manifest []string, err error) {
-	if len(exports) < 1 || f == nil {
-		return
+	if len(exports) < 1 {
+		return nil, nil
+	}
+
+	if f == nil {
+		return nil, fmt.Errorf("nil file dereference")
 	}
 
 	dirPath := g.makeExportsDir(destAbsPath)
-	if err = os.MkdirAll(dirPath, os.ModeDir|0755); err != nil {
-		return
+	if err := os.MkdirAll(dirPath, os.ModeDir|0755); err != nil {
+		return nil, err
 	}
 
 	var ok bool
@@ -631,15 +636,17 @@ func (g *Commands) export(f *File, destAbsPath string, exports []string) (manife
 	}
 
 	n := len(waitables)
-	doneAck := make(chan bool, n)
+	errsChan := make(chan error, n)
 
 	basePath := filepath.Base(f.Name)
 	baseDir := path.Join(dirPath, basePath)
 
 	for _, exportee := range waitables {
-		go func(baseDirPath, id string, urlMExt *urlMimeTypeExt) error {
+		go func(baseDirPath, id string, urlMExt *urlMimeTypeExt) {
+			var err error
+
 			defer func() {
-				doneAck <- true
+				errsChan <- err
 			}()
 
 			exportPath := sepJoin(".", baseDirPath, urlMExt.ext)
@@ -661,20 +668,21 @@ func (g *Commands) export(f *File, destAbsPath string, exports []string) (manife
 				exportURL:       urlMExt.url,
 			}
 
-			err := g.singleDownload(&dlArg)
+			err = g.singleDownload(&dlArg)
 			if err == nil {
 				manifest = append(manifest, exportPath)
 			}
-
-			return err
 		}(baseDir, f.Id, exportee)
 	}
 
 	for i := 0; i < n; i++ {
-		<-doneAck
+		ithErr := <-errsChan
+		if ithErr != nil {
+			err = reComposeError(err, ithErr.Error())
+		}
 	}
 
-	return
+	return manifest, err
 }
 
 func isLocalFile(f *File) bool {