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A shell convention is that a successful executable should exit with the value 0. Anything else can be interpreted as a failure of some sort, on part of bash or the executable you that just ran. See also $PIPESTATUS and the EXIT STATUS section of the bash man page:

   For  the shell’s purposes, a command which exits with a zero exit status has succeeded.  An exit status
   of zero indicates success.  A non-zero exit status indicates failure.  When a command terminates  on  a
   fatal signal N, bash uses the value of 128+N as the exit status.
   If  a command is not found, the child process created to execute it returns a status of 127.  If a com-
   mand is found but is not executable, the return status is 126.
   If a command fails because of an error during expansion or redirection, the exit status is greater than
   zero.
   Shell  builtin  commands  return  a  status of 0 (true) if successful, and non-zero (false) if an error
   occurs while they execute.  All builtins return an exit status of 2 to indicate incorrect usage.
   Bash itself returns the exit status of the last command executed, unless  a  syntax  error  occurs,  in
   which case it exits with a non-zero value.  See also the exit builtin command below.
It has no special meaning, other than that the last process to exit did so with an exit status of 127.
However, it is also used by bash (assuming you're using bash as a shell) to tell you that the command you tried to execute couldn't be executed (i.e. it couldn't be found). It's unfortunately not immediately deducible though, if the process exited with status 127, or if it couldn't found.
EDIT:

Not immediately deducible, except for the output on the console, but this is stack overflow, so I assume you're doing this in a script.
If you're trying to run a program using a scripting language, you may need to include the full path of the scripting language and the file to execute. For example:
exec('/usr/local/bin/node /usr/local/lib/node_modules/uglifycss/uglifycss in.css > out.css');
                Thanks, this worked for me. So I did which gs and then used the output path in my script. Worked..
– chesscov77
                Aug 2, 2017 at 16:14
This error is also at times deceiving. It says file is not found even though the files is indeed present. It could be because of invalid unreadable special characters present in the files that could be caused by the editor you are using. This link might help you in such cases.
-bash: ./my_script: /bin/bash^M: bad interpreter: No such file or directory
The best way to find out if it is this issue is to simple place an echo statement in the entire file and verify if the same error is thrown.
In addition to the given answers, note that running a script file with incorrect end-of-line characters could also result in 127 exit code if you use /bin/sh as your shell.
As an example, if you run a shell script with CRLF end-of-line characters in a UNIX-based system and in the /bin/sh shell, it is possible to encounter some errors like the following I've got after running my script named my_test.sh :
$ ./my_test.sh
sh: 2: ./my_test.sh: not found
$ echo $?
As a note, using /bin/bash, I got 126 exit code, which is in accordance with gnu.org documentation about the bash :
If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126.
Finally, here is the result of running my script in /bin/bash :
arman@Debian-1100:~$ ./my_test.sh
-bash: ./my_test.sh: /bin/bash^M: bad interpreter: No such file or directory
arman@Debian-1100:~$ echo $?
        
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