Improve Assertion Message Format

[threeal]

This issue suggests improving the assertion message format by separating the message into different lines only if it does not fit on the same line. For example, given the following case when using the string equality assertion:

• Put the message on the same line if it fits (the length is less than or equal to 75 characters):

  expected string 'some string' to be equal to 'some other string'
  

• Separate only the section that does not fit into different lines:

  expected string:
    some very very very very very very very very very very long string
  to be equal to 'some other string'
  

  expected string 'some string' to be equal to:
    some very very very very very very very very very very long string