Use `encoding-error' instead of `misc-error' for string encoding errors.

Use `encoding-error' instead of `misc-error' for string encoding errors.

* libguile/strings.c (scm_encoding_error): New function.
  (scm_from_stringn, scm_to_stringn): Use it instead of `scm_misc_error ()'.

* test-suite/lib.scm (exception:encoding-error): Adjust accordingly.

* test-suite/tests/encoding-escapes.test (exception:conversion):
  Remove.  Use `exception:encoding-error' instead.

* test-suite/tests/encoding-iso88591.test: Likewise.

* test-suite/tests/encoding-iso88597.test: Likewise.

* test-suite/tests/encoding-utf8.test: Likewise.