Report error when fails to write mock file

[maiyama18]

Close #189

This PR makes mockolo command report error in log message and finish with non-zero exit code when it fails to create mock file. Change of the log message for that situation is shown below.

before

❯ mockolo --sourcedirs Path/To/SourceDir --destination /NonexistentDirectory/Mocks.swift
Start...
["Process input mock files..."]
["Took", 4.1961669921875e-05]
["Process source files and generate an annotated/protocol map..."]
["Took", 0.023231029510498047]
["Resolve inheritance and generate unique entity models..."]
["Took", 0.0008450746536254883]
["Render models with templates..."]
["Took", 0.0002319812774658203]
["Write the mock results and import lines to", "/NonexistentDirectory/Mocks.swift"]
["Took", 0.0001609325408935547]
["TOTAL", 0.024510979652404785]
["#Protocols = 3, #Annotated protocols = 3, #Parent mock classes = 0, #Final mock classes = 3, File LoC = 77"]
["Done. Exiting program."]
Done.

after

❯ mockolo --sourcedirs Path/To/SourceDir --destination /NonexistentDirectory/Mocks.swift
Start...
["Process input mock files..."]
["Took", 6.496906280517578e-05]
["Process source files and generate an annotated/protocol map..."]
["Took", 0.07189595699310303]
["Resolve inheritance and generate unique entity models..."]
["Took", 7.808208465576172e-05]
["Render models with templates..."]
["Took", 9.894371032714844e-06]
["Write the mock results and import lines to", "/NonexistentDirectory/Mocks.swift"]
Mockolo/Executor.swift:213: Fatal error: Generation error: Error Domain=NSCocoaErrorDomain Code=4 "The folder “Mocks.swift” doesn’t exist." UserInfo={NSFilePath=/NonexistentDirectory/Mocks.swift, NSUserStringVariant=Folder, NSUnderlyingError=0x600003ce90e0 {Error Domain=NSPOSIXErrorDomain Code=2 "No such file or directory"}}
zsh: trace trap  .build/arm64-apple-macosx/debug/mockolo --sourcedirs Sources --destination

[maiyama18]

There are several options to resolve #189, but I thought that not ignoring write error is more versatile than others. Since mockolo command's primary purpose is creating mock file, it seems natural to finish whole process and report this error as the cause when failed to write mock file.

[CLAassistant]

All committers have signed the CLA.