Add check for executable of structure binary

ipyrad/analysis/structure.py

@@ -35,6 +35,14 @@
 conda install structure clumpp -c ipyrad
 """
 
+MISSING_ACCESS = """
+One of the binaries necessary for running the structure
+module are not executable. Please change the mode of the binary
+to allow execution, e.g.:
+
+chmod 777 {}
+"""
+
 _IMPORT_VCF_INFO = """
 Converting vcf to HDF5 using default ld_block_size: {}
 Typical RADSeq data generated by ipyrad/stacks will ignore this value.
@@ -194,6 +202,8 @@ def _check_binaries(self):
         for binary in [self.STRUCTURE, self.CLUMPP]:
             if not os.path.exists(binary):
                 raise IPyradError(MISSING_IMPORTS)
+            if not os.access(binary, os.X_OK):
+                raise IPyradError(MISSING_ACCESS.format(binary))
 
 
     def _setup_dirs(self):