Return M_INVALID_PARAM instead of M_BAD_JSON when setting aliases

[S7evinK]

Part of #3223 (matrix-org/matrix-spec#1286)

(For DELETE we don't validate the alias, but just return a 404 if we can't find it)

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (13c5173) 65.37% compared to head (f5bb010) 65.35%.

Files	Patch %	Lines
clientapi/routing/directory.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3297      +/-   ##
==========================================
- Coverage   65.37%   65.35%   -0.03%     
==========================================
  Files         509      509              
  Lines       57471    57471              
==========================================
- Hits        37573    37560      -13     
- Misses      16006    16019      +13     
  Partials     3892     3892              

Flag	Coverage Δ
unittests	50.01% <0.00%> (-0.02%)	⬇️

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