SI-6476 Improve error on escapement

Behavior of escape processing under string interpolation can
be confusing.  This commit improves the exception message so
you know at least what sort of escapes are handled.

This came up on SO in the form `s"\d".r`, where it may not
be obvious what is throwing and how to work around it.

```
scala> s"\d".r
scala.StringContext$InvalidEscapeException: invalid escape '\d' not one of [\b, \t, \n, \f, \r, \\, \", \'] at index 0 in "\d". Use \\ for literal \.

scala> s"a\"
scala.StringContext$InvalidEscapeException: invalid escape at terminal index 1 in "a\". Use \\ for literal \.
```

Referencing SI-6476 because that has become the magnet ticket
for "escape processing under string interpolation, huh?"

This doesn't address `$"` and doesn't handle the more interesting
parse error `s"a\"b"`.
SI-6476 Documentation

And adjust the test.
SI-6476 Unitize procedures, readability

Strictly trivial updates for readability.

I used to prefer procedure syntax, but since it was
scheduled for removal, I can't scan a def without an
equals sign.
SI-6476 Unitize ALL the procedures!
Merge pull request #3889 from som-snytt/issue/6476-alt

SI-6476 Improve error on escapement