commands: Show server error info in browser

The main item in this commit is showing of errors with a file context when running `hugo server`.

This can be turned off: `hugo server --disableBrowserError` (can also be set in `config.toml`).

But to get there, the error handling in Hugo needed a revision. There are some items left TODO for commits soon to follow, most notable errors in content and config files.

Fixes #5284
Fixes #5290
See #5325
See #5324

tpl/template_test.go

@@ -0,0 +1,31 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tpl
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestExtractBaseof(t *testing.T) {
+	assert := require.New(t)
+
+	replaced := extractBaseOf(`failed: template: _default/baseof.html:37:11: executing "_default/baseof.html" at <.Parents>: can't evaluate field Parents in type *hugolib.PageOutput`)
+
+	assert.Equal("_default/baseof.html", replaced)
+	assert.Equal("", extractBaseOf("not baseof for you"))
+	assert.Equal("blog/baseof.html", extractBaseOf("template: blog/baseof.html:23:11:"))
+	assert.Equal("blog/baseof.ace", extractBaseOf("template: blog/baseof.ace:23:11:"))
+}