fix error message on missing module command

[boegel]

reported by @robertdfrench

without this patch, in case no module commands were found in $PATH (and with $LMOD_CMD and $EASYBUILD_MODULES_TOOL undefined):

[[INFO]] Installation prefix /tmp/adfsghjm
Traceback (most recent call last):
  File "/Users/kehoste/work/easybuild-framework/easybuild/scripts/bootstrap_eb.py", line 788, in <module>
    main()
  File "/Users/kehoste/work/easybuild-framework/easybuild/scripts/bootstrap_eb.py", line 554, in main
    modtool = check_module_command(tmpdir)
  File "/Users/kehoste/work/easybuild-framework/easybuild/scripts/bootstrap_eb.py", line 208, in check_module_command
    "Known module commands are checked in order, and include: %s" % ', '.join(mod_cmds),
TypeError: sequence item 0: expected string, tuple found

with this patch:

[[INFO]] Installation prefix /tmp/adfsghjm
[[ERROR]] Could not find any module command, make sure one available in your $PATH.
Known module commands are checked in order, and include: modulecmd, lmod, modulecmd.tcl
Check the output of 'type module' to determine the location of the module command you are using.

[boegel]

@wpoely86 please review? pretty trivial though...

[wpoely86]

👍

[boegel]

Thanks for the review @wpoely86, and @robertdfrench for reporting the bug.