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

commands/server_test.go

@@ -18,6 +18,7 @@ import (
 	"net/http"
 	"os"
 	"runtime"
+	"strings"
 	"testing"
 	"time"
 
@@ -113,6 +114,18 @@ func TestFixURL(t *testing.T) {
 	}
 }
 
+func TestRemoveErrorPrefixFromLog(t *testing.T) {
+	assert := require.New(t)
+	content := `ERROR 2018/10/07 13:11:12 Error while rendering "home": template: _default/baseof.html:4:3: executing "main" at <partial "logo" .>: error calling partial: template: partials/logo.html:5:84: executing "partials/logo.html" at <$resized.AHeight>: can't evaluate field AHeight in type *resource.Image
+ERROR 2018/10/07 13:11:12 Rebuild failed: logged 1 error(s)
+`
+
+	withoutError := removeErrorPrefixFromLog(content)
+
+	assert.False(strings.Contains(withoutError, "ERROR"), withoutError)
+
+}
+
 func isWindowsCI() bool {
 	return runtime.GOOS == "windows" && os.Getenv("CI") != ""
 }