要在Angular 6的模态框中显示错误，可以按照以下步骤进行操作：

1. 创建一个错误处理服务(ErrorHandlingService)：

import { Injectable } from '@angular/core';
import { BehaviorSubject } from 'rxjs';

@Injectable()
export class ErrorHandlingService {
  private errorMessageSubject = new BehaviorSubject(null);
  errorMessage$ = this.errorMessageSubject.asObservable();

  constructor() { }

  displayError(message: string) {
    this.errorMessageSubject.next(message);
  }
}

2. 在模态框组件中注入错误处理服务：

import { Component, OnInit } from '@angular/core';
import { NgbModal, ModalDismissReasons } from '@ng-bootstrap/ng-bootstrap';
import { ErrorHandlingService } from 'path-to-error-handling-service';

@Component({
  selector: 'app-modal',
  templateUrl: './modal.component.html',
  styleUrls: ['./modal.component.css']
})
export class ModalComponent implements OnInit {
  closeResult: string;

  constructor(private modalService: NgbModal, private errorHandlingService: ErrorHandlingService) { }

  ngOnInit() {
    this.errorHandlingService.errorMessage$.subscribe(message => {
      if (message) {
        this.openModal(message);
      }
    });
  }

  open(content) {
    this.modalService.open(content, { ariaLabelledBy: 'modal-basic-title' }).result.then((result) => {
      this.closeResult = `Closed with: ${result}`;
    }, (reason) => {
      this.closeResult = `Dismissed ${this.getDismissReason(reason)}`;
    });
  }

  private openModal(message: string) {
    this.modalService.open(ModalContentComponent, { size: 'lg' }).componentInstance.message = message;
  }

  private getDismissReason(reason: any): string {
    if (reason === ModalDismissReasons.ESC) {
      return 'by pressing ESC';
    } else if (reason === ModalDismissReasons.BACKDROP_CLICK) {
      return 'by clicking on a backdrop';
    } else {
      return `with: ${reason}`;
    }
  }
}

3. 创建一个用于显示错误信息的模态框内容组件(ModalContentComponent)：

import { Component, Input } from '@angular/core';
import { NgbActiveModal } from '@ng-bootstrap/ng-bootstrap';

@Component({
  selector: 'app-modal-content',
  template: `
    

      
Error
      
        ×
      
    
    

      {{ message }}
    
    

      Close
    
  `
})
export class ModalContentComponent {
  @Input() message: string;

  constructor(public activeModal: NgbActiveModal) { }
}

4. 在需要显示错误的地方，调用错误处理服务的displayError方法：

import { Component } from '@angular/core';
import { ErrorHandlingService } from 'path-to-error-handling-service';

@Component({
  selector: 'app-root',
  template: `
    Trigger Error
    
  `,
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  constructor(private errorHandlingService: ErrorHandlingService) { }

  triggerError() {
    this.errorHandlingService.displayError('Something went wrong');
  }
}

通过以上步骤，当调用triggerError()方法时，错误处理服务将显示一个带有错误消息的模态框。