Error messages should be in the format of what is and what can or cannot be done. Use a period in error message text, even if there is only one sentence. If there are multiple sentences, use only one space after the periods, not two.
Should exception message end with period?
From Best Practices for Exceptions† on MSDN in the section “Creating and raising exceptions”: Use grammatically correct error messages, including ending punctuation. Each sentence in a description string of an exception should end in a period.
Should tooltips have periods?
Periods and Other Terminal Punctuation In general, do not use punctuation at the end of a headline, button label, checkbox or radio button label, tooltip, text link (unless the link ends a sentence), or navigation element.
Should error messages be capitalized?
Error strings should not be capitalized (unless beginning with proper nouns or acronyms) or end with punctuation, since they are usually printed following other context.
