Make all exception classes inherit from std::exception

This refactors out a very common pattern of formatting "%s: %s" with
e.function.c_str() and strerror( e.the_errno ) into just the what()
method of NetworkException. It's also a prerequisite for making cleaner
public API for any exceptions we throw, and allows us to more easily
get exceptions passed back to us to handle.

src/tests/encrypt-decrypt.cc

@@ -144,7 +144,7 @@ int main( int argc, char *argv[] ) {
       test_one_session();
     } catch ( const CryptoException& e ) {
       fprintf( stderr, "Crypto exception: %s\r\n",
-               e.text.c_str() );
+               e.what() );
       fatal_assert( false );
     }
   }