This improves the following situation, in which '|' is the cursor position:
```yaml
- key1: value1
key2: value2|
```
Before this commit, by pressing Enter would result in this indentation:
```yaml
- key1: value1
key2: value2
|
```
Which is less convenient than this:
```yaml
- key1: value1
key2: value2
|
```
This helps to solve the first issue reported at #1377.
The problem is that the line loses its indentation when we start
inserting a new mapping pair (specifically, when we enter the character
":").
The issue seems to be solved by auto indenting ERROR nodes.
This solves the second issue reported at #1377, in which the cursor
would not be aligned with the line "key2: value2" when entering a new
line below it.
```yaml
- key1: value1
key2: value2
```
