Improve the default failure message for custom matchers

[kana]

Derived from gh-14.

Adding to this, I think it should also be documented that the arguments are implicitly a list. Consider this test:

function! ToBeInteger(actual, expected)
  return a:actual == a:expected
endfunction

call vspec#customize_matcher('to_be_int', {'match': function('ToBeInteger')})

describe 'vspec default failure message'
  it 'has undocumented representation'
    let fortytwo = 42
    Expect fortytwo to_be_int 43
  end
end

This yields a message stating 42 does not equal [43], which may be misleading.

not ok 1 - vspec default failure message has undocumented representation
# Expected fortytwo to_be_int 43
#       Actual value: 42
#     Expected value: [43]