Replace regex-based error message tests with implementation notes

Several exercise test suites were using the assertRaisesRegexp method
to test whether errors raised by the solution are worded in a specific
way. As discussed in #30 these tests are changed to use the assertRaises
method and short written implementation notes are added to the test
suites that ask for meaningful error messages.

The following exercises are affected:

  * largest-series-product
  * nucleotide-count
  * octal
  * saddle-points
  * series

Fixes #30.