Spec for error_stop and check

src/stdlib_experimental_error.md

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+# Catching and handling errors
+
+* [`check` - Checks the value of a logical condition](#check---checks-the-value-of-a-logical-condition)
+* [`error_stop` - aborts the program](#error_stop---aborts-the-program)
+
+
+## `check` - Checks the value of a logical condition
+
+### Description
+
+Checks the value of a logical condition.
+
+### Syntax
+
+`call check(condition, msg, code, warn)`
+
+### Arguments
+
+`condition`: Shall be a scalar of type `logical`.
+
+`msg` (optional): Shall be a character expression containing the message to be printed to `stderr`. The default `msg` is 'Check failed.'.
+
+`code` (optional): Shall be a scalar of type `integer`. The default `code` is `1`.
+
+`warn` (optional): Shall be a scalar of type `logical`. The default `warn` is `.true.`.
+
+### Return value
+
+If `condition` is `.false`., and:
+
+ * no other arguments are provided, this subroutine stops the program with the default message and exit code 1;
+
+ * `msg` is provided, this subroutine stops the program and it prints the value of `msg`;
+
+ * `code` is provided, this subroutine stops the program with the given exit code;
+
+ * `warn` is provided and `warn` is `.true.`, this subroutine doesn't stop the program and prints the message.
+
+### Examples
+
+```fortran
+program demo_check1
+    use stdlib_experimental_error, only: check
+    implicit none
+    integer :: a = 1
+    ! If a /= 5, stops the program with exit code 1 and prints 'Check failed.'
+    call check(a == 5)
+end program demo_check1
+```
+```fortran
+program demo_check2
+    use stdlib_experimental_error, only: check
+    implicit none
+    integer :: a = 1
+    ! If a /= 5, stops the program with exit code 1 and prints  'a == 5 failed.'
+    call check(a == 5, msg='a == 5 failed.')
+end program demo_check2
+```
+```fortran
+program demo_check3
+    use stdlib_experimental_error, only: check
+    implicit none
+    integer :: a = 1
+    ! If a /= 5,  prints 'a == 5 failed.', but doesn't stop the program.
+    call check(a == 5, msg='a == 5 failed.', warn=.true.)
+end program demo_check2
+```
+```fortran
+program demo_check3
+    use stdlib_experimental_error, only: check
+    implicit none
+    integer :: a = 1
+    ! If a /= 5, stops the program with exit code 77 and prints 'a == 5 failed.'
+    call check(a == 5, msg='a == 5 failed.', code=77)
+end program demo_check3
+```
+
+## `error_stop` - aborts the program
+
+### Description
+
+Aborts the program with a message and a nonzero exit code.
+
+### Syntax
+
+`call error_stop(msg, code)`
+
+### Arguments
+
+`msg`: Shall be a character expression containing the message to be printed to `stderr`.
+
+`code` (optional): Shall be a scalar of type `integer` to be returned as exit code.
+
+### Output
+
+Aborts the program with printing the message `msg` to `stderr` and a nonzero exit code. The nonzero exit code is equal to `code` if provided, and 1 otherwise.
+
+### Example
+
+```fortran
+program demo_error_stop
+    use stdlib_experimental_error, only: error_stop
+    implicit none
+    call error_stop("Invalid argument", code = 123)
+end program demo_error_stop
+```