expose SchemaError in init

[tcchiao]

this allows usage such as from xarray_schema import SchemaError instead of from xarray_schema.base import SchemaError

[codecov]

Codecov Report

Merging #14 (38f116f) into main (d21f715) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #14   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          258       259    +1     
=========================================
+ Hits           258       259    +1     

Impacted Files	Coverage Δ
xarray_schema/__init__.py	100.00% <100.00%> (ø)

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