Distinguish between missing semicolon and extra tokens

[outoftime]

This stylesheet:

p {
   padding: 1px 1px 1px 1px 1px;
}

Yields an “extra tokens after value” error from PrettyCSS. This is
what we’d expect, since there is an extra 1px at the end of the value,
which is not a valid way to write padding.

However, this will generate the same error:

p {
   padding: 1px
   margin: 4px;
}

Since CSS is insensitive to newlines, the margin on the second line
appears to be a continuation of the value for padding on the first
line.

We can heuristically identify situations where the “extra tokens after
value” error is the result of a missing semicolon:

• The token that fails to parse is the first non-whitespace string on
  its line
• The previous line does not end in a semicolon

So, we now get good error messages for each of these situations: