fix: add devmode warning for removing fragment markers

[mlrawlings]

Description

This adds additional checks in dev mode to give more context to the user as to why diffing failed when the DOM has been updated manually and fragment markers (empty text nodes) were removed.

Instead of throwing TypeError: Cannot read property 'nextSibling' of null, it now throws Error: Cannot get fragment nodes. Boundary markers missing.. In addition to the new error text, a second error is passed to console.warn showing the location where the fragment marker was removed.

Checklist:

• I have read the CONTRIBUTING document and have signed (or will sign) the CLA.
• I have updated/added documentation affected by my changes.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Merging #1541 into master will decrease coverage by 0.05%.
The diff coverage is 62.50%.

@@            Coverage Diff             @@
##           master    #1541      +/-   ##
==========================================
- Coverage   90.58%   90.52%   -0.06%     
==========================================
  Files         356      356              
  Lines       12707    12731      +24     
==========================================
+ Hits        11510    11525      +15     
- Misses       1197     1206       +9     

Impacted Files	Coverage Δ
...kages/marko/src/runtime/components/util-browser.js	89.01% <42.85%> (-8.40%)	⬇️
...ckages/marko/src/runtime/vdom/morphdom/fragment.js	93.47% <66.66%> (-1.88%)	⬇️
packages/marko/src/runtime/components/util.js	92.30% <100.00%> (+1.00%)	⬆️
packages/marko/src/runtime/vdom/morphdom/index.js	98.09% <100.00%> (+0.02%)	⬆️

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