fix: Ensures provided packageFiles arguments are merged with bumpFiles when no bumpFiles argument is specified (default).

[jbottigliero]

• Merges packageFiles with defaults.bumpFiles before merging in arguments when a packageFiles argument is present.
• Adds some simple "validation" to updaters and improves error messaging.
• Adds a test case covering Question: Are packageFiles in .versionrc not bumped automatically? #533

closes #533

[coveralls]

Coverage decreased (-0.09%) to 99.343% when pulling 1110293 on issue-533 into eb11776 on master.

[joejordanbrown]

@jbottigliero

Just faced this issue.

Is there a reason why this wasn't merged?

[codecov-commenter]

Codecov Report

Merging #534 (97af9a8) into master (a0bc835) will decrease coverage by 0.24%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #534      +/-   ##
==========================================
- Coverage   97.73%   97.49%   -0.25%     
==========================================
  Files          25       25              
  Lines        1106     1118      +12     
==========================================
+ Hits         1081     1090       +9     
- Misses         25       28       +3     

Impacted Files	Coverage Δ
lib/updaters/index.js	77.14% <71.42%> (-7.24%)	⬇️
index.js	100.00% <100.00%> (ø)
test/core.spec.js	98.11% <100.00%> (+0.03%)	⬆️

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