Typos, capitalization

For exceptions whose text is a single statement, I've updated each to use sentence capitalization but no terminating period; these may logically stand on their own in an error display without ambiguity. For messages whose content is two distinct fragments (those with an error, then "Valid encodings are..."), I have updated them to be two actual sentences, each terminated with a period to avoid ambiguity.
Merge pull request #154 from redler/patch-3

Typos, capitalization