Addresses issue - [Differentiate fault and response unmarshal error]

[AjithPanneerselvam]

Issue link: #63

What does this PR offer?

Now, we can differentiate the fault error from other errors we get from *Response.Unmarshal(). The sample code is given below.

        var sampleResponse SampleResponse
	err = response.Unmarshal(&sampleResponse)
	if err != nil {
		if gosoap.IsFault(err) {
			return "", errors.Wrap(err, "error as fault occurred")  // Or, do something instead of returning err
		}

		return "", errors.Wrap(err, "error unmarshalling sample response")
	}

All the unit tests passed.

[codecov-commenter]

Codecov Report

Merging #64 into master will decrease coverage by 0.29%.
The diff coverage is 62.50%.

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
- Coverage   74.42%   74.13%   -0.30%     
==========================================
  Files           5        5              
  Lines         305      317      +12     
==========================================
+ Hits          227      235       +8     
- Misses         63       67       +4     
  Partials       15       15              

Impacted Files	Coverage Δ
response.go	55.55% <57.14%> (+5.55%)	⬆️
wsdl.go	100.00% <100.00%> (ø)

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[AjithPanneerselvam]

@tiaguinho Please review the PR.