Failed packrat::init() leaves broken .Rprofile

When running packrat::init() on an existing project, it is not uncommon to discover that a package used in the project is not installed locally.

> packrat::init()
Initializing packrat project in directory:
- "~/my_project"
Error: Unable to retrieve package records for the following packages:
- 'Biobase'
In addition: Warning messages:
1: In FUN(X[[i]], ...) :
  Package 'Biobase' not available in repository or locally

At this stage, the most natural thing to do is to install the missing package, but the installation will fail because source("packrat/init.R") is now included in the project's .Rprofile even though the packrat/ directory doesn't exist. Furthermore, if there are multiple missing packages, install.packages() doesn't stop on error but tries to download the full package list and then those packages' installations individually fail.

> BiocManager::install("Biobase")
Bioconductor version 3.7 (BiocManager 1.30.1), R 3.5.0 (2018-04-23)
Installing package(s) 'Biobase'
also installing the dependency ‘BiocGenerics’

trying URL 'https://bioconductor.org/packages/3.7/bioc/src/contrib/BiocGenerics_0.26.0.tar.gz'
Content type 'application/x-gzip' length 45638 bytes (44 KB)
==================================================
downloaded 44 KB

trying URL 'https://bioconductor.org/packages/3.7/bioc/src/contrib/Biobase_2.40.0.tar.gz'
Content type 'application/x-gzip' length 1657220 bytes (1.6 MB)
==================================================
downloaded 1.6 MB

Error in file(filename, "r", encoding = encoding) : 
  cannot open the connection
Calls: source -> file
In addition: Warning message:
In file(filename, "r", encoding = encoding) :
  cannot open file 'packrat/init.R': No such file or directory
Execution halted
Error in file(filename, "r", encoding = encoding) : 
  cannot open the connection
Calls: source -> file
In addition: Warning message:
In file(filename, "r", encoding = encoding) :
  cannot open file 'packrat/init.R': No such file or directory
Execution halted
... even more output ...

It may be better to condition the sourcing on the existence of packrat/

if (dir.exists("packrat")) source("packrat/init.R")

or to possibly defer updating .Rprofile until init() succeeds, or to restore the old .Rprofile if init() fails.

Edit: I'd be happy to submit a PR for any of the above options if desired.

Start at packrat::init() and inspect how it updates the project's .Rprofile and creates packrat/init.R. Reproduce the missing-package failure; done means a failed init does not leave .Rprofile sourcing a nonexistent packrat/init.R, and subsequent package installation no longer triggers that error.