SimpleTemplate 0.84, 92 bytes

This challenge was simple, yet fun!

Simply checks if the input is just "hhh...." and outputs the text, or throws an exception with the content "err".

{@ifargv.0 matches"@^h+$@"M}{@eachM.0}Hello World{@/}{@else}{@phpthrow new Exception("err")}

There is no built-in to throw an exception, so, I had to resort to PHP.

If just outputting err to STDOUT is fine, this can be reduced to 63 bytes:

{@ifargv.0 matches"@^h+$@"M}{@eachM.0}Hello World{@/}{@else}err

Ungolfed:

Below is a more readable version of the code:

{@if argv.0 matches "@^h+$@"}
    {@each argv.0 as h}
        {@echo "Hello World"}
    {@/}
{@else}
    {@php throw new Exception("err")}
{@/}

You can try this on: http://sandbox.onlinephpfunctions.com/code/0f87b846cadde59a01a9fae12b924ce3a406a4c6

SimpleTemplate 0.84, 92 63 bytes

This challenge was simple, yet fun!

Simply checks if the input is just "hhh...." and outputs the text, or outputs "err" to STDOUT:

{@ifargv.0 matches"@^h+$@"M}{@eachM.0}Hello World{@/}{@else}err

The big byte saving was due to the-cobalt's comment:

Outputting to STDOUT is fine, so you could use your 63 byte version.

Ungolfed:

Below is a more readable version of the code:

{@if argv.0 matches "@^h+$@"}
    {@each argv.0 as h}
        {@echo "Hello World"}
    {@/}
{@else}
    {@echo "err"}
{@/}

You can try this on: http://sandbox.onlinephpfunctions.com/code/e35a07dfbf6b3b56c2608aa86028b395ef457129