Reduce duplication in text-only UI and TUI (#565)

This introduces the concept of a `batchExecUI`: the UI that displays the
status of a batch spec execution.

Both `batchExecTUI` and `batchExecJSONLinesUI` implement that interface
and can thus be used interchangeably.

The result is that we can get rid of the duplicated code paths.

Note: this is not as nice as it could be, because I want to be careful
and only change what is necessary in this PR to get rid of the
duplication (vs. rethinking how we batch spec execution reports
progress, etc.) So please review from that point of view.

Author: mrnugget

cmd/src/batch_apply.go

@@ -41,32 +41,24 @@ Examples:
 		ctx, cancel := contextCancelOnInterrupt(context.Background())
 		defer cancel()
 
-		var err error
+		var ui batchExecUI
 		if flags.textOnly {
-			err = textOnlyExecuteBatchSpec(ctx, executeBatchSpecOpts{
-				flags:  flags,
-				client: cfg.apiClient(flags.api, flagSet.Output()),
-
-				applyBatchSpec: true,
-			})
-			if err != nil {
-				fmt.Printf("ERROR: %s\n", err)
-				return &exitCodeError{nil, 1}
-			}
+			ui = &batchExecJSONLinesUI{}
 		} else {
 			out := output.NewOutput(flagSet.Output(), output.OutputOpts{Verbose: *verbose})
-			err = executeBatchSpec(ctx, executeBatchSpecOpts{
-				flags:  flags,
-				out:    out,
-				client: cfg.apiClient(flags.api, flagSet.Output()),
-
-				applyBatchSpec: true,
-			})
-			if err != nil {
-				printExecutionError(out, err)
-				out.Write("")
-				return &exitCodeError{nil, 1}
-			}
+			ui = &batchExecTUI{out: out}
+		}
+
+		err := executeBatchSpec(ctx, executeBatchSpecOpts{
+			flags:  flags,
+			client: cfg.apiClient(flags.api, flagSet.Output()),
+
+			applyBatchSpec: true,
+
+			ui: ui,
+		})
+		if err != nil {
+			return &exitCodeError{nil, 1}
 		}
 
 		return nil