Escape control characters and most whitespace in error tracebacks

When a ParseError is raised and printed, the pattern it tried to match is printed as well. If the pattern has control characters like `\n` or `\r`, these will get printed and make the output awkward to read.

The problem is not so bad when the only whitespace characters in the pattern are a regular space and a tab:

```pycon
>>> p = pe.compile('"a" [ \t] "b"')
>>> p.match("ab")
Traceback (most recent call last):
[...]
pe._errors.ParseError:
line 0, character 0
ab
^
ParseError: `a[\ \ ]b`
```

But when a `\r` is part of the pattern, the `ParseError` name gets overwritten:

```pycon
>>> p = pe.compile('"a" [ \t\r] "b"')
>>> p.match("ab")
Traceback (most recent call last):
[...]
pe._errors.ParseError:
line 0, character 0
ab
^
]b`seError: `a[\ \ \
```

The characters should be escaped so it looks more like this:

```
ParseError: `a[ \t\r]b`
```

Also see #59.