py: Small changes to objstr.c, including a bug fix.

Some small fixed:

- Combine 'x' and 'X' cases in str format code.

- Remove trailing spaces from some lines.

- Make exception messages consistently begin with lower case (then
needed to change those in objarray and objtuple so the same
constant string data could be used).

- Fix bug with exception message having %c instead of %%c.