Include more characters in a Unicode warning

[vdancik]

When issuing a warning about Unicode characters (#41), include few characters before/after the offending symbol - frequently it is a space (nbsp) or a dash (minus) and is difficult to locate.

[jagus7]

As Zhou mentioned during the DB WG teleconference in May, the documentation should be developed to check the length violations and verify all the fields.

[zhouji2013]

The warning message containing the 'noncompliant' character is improved this way: the noncompliant character is shown along with up tp 5 characters before and after, respectively; the noncompliant character itself is displayed in red color.

Notice that for the sake of efficiency, once the first noncompliant character is found, the checking process stops. So if there are multiple consecutive noncompliant characters, only the first one is displayed in red.