Improve logging when no functions were bundled

[ehmicky]

This improves the log message when build.functions was specified but no functions were found/bundled.

[codecov]

Codecov Report

Merging #505 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #505      +/-   ##
========================================
- Coverage   91.01%    91%   -0.01%     
========================================
  Files          47     47              
  Lines        1079   1078       -1     
  Branches      121    122       +1     
========================================
- Hits          982    981       -1     
  Misses         74     74              
  Partials       23     23

Flag	Coverage Δ
#macOS	90.53% <100%> (-0.01%)	⬇️
#node_13	91% <100%> (-0.01%)	⬇️
#node_8_3_0	90.53% <100%> (-0.01%)	⬇️
#ubuntu	90.53% <100%> (-0.01%)	⬇️
#windows	86.45% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
packages/build/src/plugins/functions/index.js	94.59% <100%> (-0.15%)	⬇️

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