Enhances Helmfile to print more helpful message on error while calling `exec` template function.
Helmfile has been printing error messages like the below:
```
in ./helmfile.yaml: error during helmfile.yaml.part.0 parsing: template: stringTemplate:5:8: executing "stringTemplate" at <exec "./exectest.sha" (list)>: error calling exec: exit status 1
```
Adding captured stdout and stderr, with some indentation to make it readable, it now produces the following message on missing executable:
```
$ make build && ./helmfile build
go build
in ./helmfile.yaml: error during helmfile.yaml.part.0 parsing: template: stringTemplate:5:8: executing "stringTemplate" at <exec "./exectest.sha" (list)>: error calling exec: fork/exec ./exectest.sha: no such file or directory
COMMAND:
./exectest.sha
ERROR:
fork/exec ./exectest.sha: no such file or directory
```
On non-zero exit status without output:
```
$ make build && ./helmfile build
go build
in ./helmfile.yaml: error during helmfile.yaml.part.0 parsing: template: stringTemplate:5:8: executing "stringTemplate" at <exec "./exectest.sh" (list)>: error calling exec: exit status 1
COMMAND:
./exectest.sh
ERROR:
exit status 1
```
On non-zero exit status with output:
```
$ make build && ./helmfile build
go build
in ./helmfile.yaml: error during helmfile.yaml.part.0 parsing: template: stringTemplate:5:8: executing "stringTemplate" at <exec "./exectest.sh" (list)>: error calling exec: exit status 2
COMMAND:
./exectest.sh
ERROR:
exit status 2
COMBINED OUTPUT:
out1
err1
```
Resolves#1158
KUOKA Yusuke
a-hat