Capitalization and punctuation of error messages

[dtolnay]

Libraries seem to be all over the place. Can we standardize on lowercase with no period at the end?

[dtolnay]

The standard library uses lowercase without trailing punctuation.

"unexpected end of file"

"provided string was not `true` or `false`"

"invalid IP address syntax"

"second time provided was later than self"

"invalid utf-8 sequence of {} bytes from index {}"

"environment variable was not valid unicode: {:?}"

[mehcode]

In line with this, capitalization and punctuation of expect should be discussed. I'm in favor of it being treated the same, however, the std library suggests expect messages should be capitalized while at the same time using lowercase anywhere that I looked.

[dtolnay]

Added in 201d903.