MSH2 and head and neck squamous cell carcinoma: MLH1 rs1800734, MSH2 rs2303426, MSH3 rs26279, EXO1 rs1047840 and EXO1 rs9350 single nucleotide polymorphism genotypes and EXO1 haplotypes in 90 patients with head and neck squamous cell carcinoma stratified by toxicity and response rate to concurrent chemoradiotherapy