Made TextException a child of std::exception so that it is easier to catch

Made TextException a child of std::exception so that it is easier to catch
more exceptions (standard and custom) with one catch(). The catching code
usually does not care what the exception is anyway.

TextException needs more work to report more information in what() method.

Catch std::exception to catch more printable exceptions. TextException is an
std::exception [child].

These changes were inspired by and required for eCAP.