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/examples/ntester.cc

@@ -66,7 +66,7 @@ int main( int argc, char *argv[] )
       n = new Transport<UserStream, UserStream>( me, remote, NULL, NULL );
     }
   } catch ( const CryptoException &e ) {
-    fprintf( stderr, "Fatal error: %s\n", e.text.c_str() );
+    fprintf( stderr, "Fatal error: %s\n", e.what() );
     exit( 1 );
   }
 
@@ -98,7 +98,7 @@ int main( int argc, char *argv[] )
 	  }
 	}
       } catch ( const CryptoException &e ) {
-	fprintf( stderr, "Cryptographic error: %s\n", e.text.c_str() );
+	fprintf( stderr, "Cryptographic error: %s\n", e.what() );
       }
     }
   } else {
@@ -164,10 +164,10 @@ int main( int argc, char *argv[] )
 	  n->recv();
 	}
       } catch ( const NetworkException &e ) {
-	fprintf( stderr, "%s: %s\r\n", e.function.c_str(), strerror( e.the_errno ) );
+	fprintf( stderr, "%s\n", e.what() );
 	break;
       } catch ( const CryptoException &e ) {
-	fprintf( stderr, "Cryptographic error: %s\n", e.text.c_str() );
+	fprintf( stderr, "Cryptographic error: %s\n", e.what() );
       }
     }