Change BlownUpMat to return fail for certain invalid inputs

Specifically, return fail if Coefficients() returns fail.

Fixes #826

lib/matrix.gi

@@ -3103,7 +3103,9 @@ InstallGlobalFunction( BlownUpMat, function ( B, mat )
       for b in vectors do
         resrow:= [];
         for entry in row do
-          Append( resrow, Coefficients( B, entry * b ) );
+          entry := Coefficients( B, entry * b );
+          if entry = fail then return fail; fi;
+          Append( resrow, entry );
         od;
         ConvertToVectorRepNC( resrow );
         Add( result, resrow );

tst/testbugfix/2017-07-09-blowup.tst

@@ -0,0 +1,18 @@
+# Fixes https://github.com/gap-system/gap/issues/826
+# Modified BlownUpMat to return fail for invalid input
+gap> BlownUpMat(Basis(CF(4)), [[1]]);
+[ [ 1, 0 ], [ 0, 1 ] ]
+gap> BlownUpMat(Basis(CF(4)), [[E(4)]]);
+[ [ 0, 1 ], [ -1, 0 ] ]
+gap> BlownUpMat(Basis(CF(4)), [[Sqrt(2)]]);
+fail
+
+# Simplified version of example from issue #826
+gap> i:=E(4);;
+gap> S:=((1+i)/Sqrt(2))*DiagonalMat([i,i,1,1]);;
+gap> T:=((1+i)/2)*[[-i,0,0,i],[0,1,1,0],[1,0,0,1],[0,-i,i,0]];;
+gap> g:=Group(T);;
+gap> Order(g);
+10
+gap> S in g;
+false