[ADQL] Throwing a ParseException instead of an Error

when an incorrect character that can not be interpreted by
the JavaCC Token Manager is encountered.

Actually, the TokenMgrError thrown by JavaCC is caught by all
ADQLParser.parseQuery(...) functions, wrapped inside a ParseException
which is finally thrown instead of the TokenMgrError. In this way,
ADQL-Lib users just have to care about a single Throwable:
ParseException.

Besides the error message has been slightly modified from:

> Lexical error at line 1, column 10.  Encountered: "\u00e9" (233), after : \"\"

to:

> Incorrect character encountered at l.1, c.10: \"\\u00e9\" ('é'), after : \"\"

Thus, the error is more user-friendly, more easy to understand by users.
Additionally, the incorrect character is displayed, as before, in its unicode
expression, but also in its character form (instead of an integer value that
nobody can really understand).

This commit fixes the GitHub issue #17