Change mock request to return empty json

Previously the mock returned an empty string as the request body. This
was converted to nil which breaks the (probably valid) assumption that
ErrorResponse#error makes when it calls @body["errorMessage"], that
@body will not be nil.
include error message when raising BadSyntax

Pass on error message from the ArangoDB server when raising BadSyntax
errors:

db.query.execute('FOR t IN TRAVERSE(imdb_vertices,
imdb_edges, "imdb_vertices/759", "outbound", {maxDepth: 2}) FILTER
t.vertex.genre == @foo RETURN t', bindVars: {"foo" => "Comedy" }).to_a

Ashikawa::Core::BadSyntax: 1540: usage of unknown function 'TRAVERSE()'
Merge pull request #110 from sleepycat/syntax_exception

Pass on server error messages when raising BadSyntax exceptions
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